World War Two: Marianas(2-4); Prewar Japanese Activities

At the termination of World War I Japan, as one of the Allied powers, was awarded a Class C mandate over all of the islands and atolls north of the equator that had formerly been in the possession of the German Empire. These included the Marshalls, the Carolines, the Palaus, and the Marianas except for Guam. Under the terms of Article 22 of the Covenant of the League of Nations, Japan agreed to refrain from “the establishment of fortifications or military and naval bases” in her newly acquired territories. Eleven years later Japan gave the required two years notice of her intention to withdraw from the League and did so officially on 27 March 1935.

Whether or not the Japanese made any active effort to fortify or garrison the mandates before 1933 remains in doubt, although their policy of excluding foreign visitors from these scattered islands inevitably raised suspicions in the minds of interested westerners as to what was going on behind the silken curtain. In any event after her withdrawal from the League and before the outbreak of hostilities it is certain that Japan embarked on a program of military construction in the area. This was done in spite of the fact that Japan’s secession from the League was a unilateral act and in international law did not relieve her of accepted obligations under the League Covenant.

From 1934 to 15 November 1940, the Japanese Government is known to have appropriated at least 14,456,800 yen ($3,939,478) for construction in the Marianas alone.3 For the year beginning 15 November 1940, a minimum of 121,189,666 yen ($28,406,858) was appropriated for construction in the Marianas, Carolines, and Marshalls, of which 15,605,885 yen ($3,658,019) was allocated to the Marianas. Of the total of over thirty million yen ($7,032,000) spent in the Marianas, about 60 percent was assigned to Saipan, 25 percent to Tinian, and 15 percent to Pagan. About 40 percent of the Saipan appropriation was for air installations, the remainder being allocated to fortifications, barracks, storage buildings, offices, water supply facilities, ammunition storage facilities, and communications stations. Practically all of the money allocated to Tinian and Pagan was earmarked for airfield construction.

In 1934 work began on Aslito airfield, located near the southern end of Saipan, Aslito was the principal Japanese air base in the Marianas, and its capture and development was to be the main objective of the American forces that invaded the Marianas in 1944. In addition to Aslito, a seaplane base in Tanapag Harbor, Saipan, was completed in 1935. Five years later nearby Tinian could boast an airfield costing about 7.5 million yen ($1,758,000).

Although various Japanese spokesmen after the close of World War II pretended that these airfields and other building activities in the mandates were undertaken for peaceful purposes, the pretense was a flimsy one. A close examination of appropriations made by the Japanese Government for construction in the Marianas in 1940 and 1941 clearly indicates that Japan had launched an active program of military fortification of the mandates well before the actual outbreak of hostilities. In November of 1940 a sizable appropriation was made for the construction of “lighthouses” throughout the mandated islands. Each “lighthouse” came equipped with barracks, ammunition storage facilities, a command post, and a lookout station. Actually, of course, these were naval lookout stations.

On Saipan, construction was fairly extensive. Twelve “lighthouses” were constructed at a cost of 1,333,333 yen ($312,533). In February 1941, 100,000 yen ($23,440) was set aside to build four gun positions of reinforced concrete, to be completed by the end of July. During 1941 almost 700,000 yen ($164,080) was devoted to the construction of the Saipan branch of the 4th Fleet Naval Stores Department, including ammunition storage sheds with a floor area of 800 square meters. Also during 1941, almost 800,000 yen ($187,520) was earmarked for construction of communications facilities, including receiving and sending stations, radio direction finders, and barracks for the personnel to man them. In September 1941, 1,500,000 yen ($351,600) was devoted to building military barracks, baths and latrines, kitchens, infirmaries, storehouses, workshops, torpedo storage sheds, garages, and air raid shelters. The order authorizing this expenditure specifically stated that these structures were intended for the use of a base force and a defense force, both of which under Japanese naval organization were acknowledged combat units.

There is other evidence to prove that in the year or two before Pearl Harbor Japan was making active preparations to use the mandates, including the Marianas, as military and naval bases, contrary to the terms of the League Covenant.

On 15 November 1939 the 4th Fleet of the Imperial Japanese Navy was organized and placed in charge of garrisons and fortifications in the mandates.6 This fleet, which was primarily a base defense unit rather than the more orthodox type of naval combat unit, established its headquarters at Truk. For administrative and defense purposes, the mandates were divided into four sectors—East Carolines, West Carolines, Marshalls, and Marianas, with sector headquarters at Truk, the Palaus, Kwajalein, and Saipan, respectively. Each sector was controlled by a base force subordinate to the 4th Fleet, and each base force commanded subordinate shore and surface units within its own sector.

Shortly after the creation of the 4th Fleet, the 5th Special Base Force was activated in Japan and assigned the duty of preparing for the fortification and defense of the Marianas. Attached to it were the 5th Communications Unit and the 5th Defense Force, the latter unit comprising the bulk of the combat personnel located in this area before the outbreak of war with the United States. These troops arrived in the Marianas in December of 1940. Not long afterward their strength was augmented by a detachment each of the 4th Fleet Naval Stores Department and the 4th Naval Air Depot, both located on Saipan.

The mission of the 5th Special Base Force from December 1940 to 31 May 1941 was to defend its assigned areas and speed up preparations for combat in the event of a war. The subordinate 5th Defense Force engaged in construction of gun positions, road building, harbor improvement, and sundry other duties aimed at enhancing the security of Saipan. During the succeeding period, from 1 June through 30 November, an additional mission was assigned to the 5th Special Base Force—that of “planning and preparation for the Guam invasion operation.” No additional evidence need be adduced to show that well before the Pearl Harbor attack, Japan had committed herself unequivocally to a policy of fortifying the Marianas for offensive as well as defensive purposes.

From Pearl Harbor to Invasion

For the first two years after the Pearl Harbor attack, the fighting war between Japan and the United States remained far from the shores of Saipan and her sister islands except, of course, for the Japanese invasion of Guam. The Marianas during this period served the Japanese chiefly as supply and staging bases for troops, ships, and planes engaged in battle well to the east and south, and the strength of combat naval shore units in the area remained low.

The 5th Special Base Force on Saipan ranged from a low of 919 military troops and 220 civilians in May 1943 to a total of 1,437 men in February of the following year. The 54th Naval Guard Force on Guam had 302 men in September 1942, and in early 1944 received an additional 425 recruits. In September 1943 the Yokosuka 1st Special Naval Landing Force, numbering about 1,500 men, arrived on Saipan, but in the following January was reduced by about a third when detachments were sent to the Rabaul area.

Considering the magnitude of the Empire’s troop commitments elsewhere, the garrison assigned to the Marianas can be considered no more than a token force.

This is not at all surprising. The Marianas were a rear area. Simple military logic dictated that the Japanese concentrate their efforts in the Rabaul-New Guinea area to the south and build up the defenses of the Gilberts and Marshalls to the east. There was not enough money, manpower, or matériel to build strong fortifications and defenses on every one of Japan’s myriad island possessions, and those farther away from the direct line of American advance had to suffer neglect. Not until the Marshall Islands finally fell to the Americans did the position of the Marianas become dangerous enough to justify urgent measures in their defense.

Reinforcement of the Marianas By February 1944 the Marianas garrisons could predict that their time had come. Tarawa and Makin in the Gilberts had been captured by U.S. forces in November 1943. Kwajalein Atoll in the Marshalls fell in early February 1944, and Eniwetok, less than a thousand nautical miles from Saipan, in mid-February. Also in mid-February, Admiral Mitscher’s Fast Carrier Task Force (Task Force 58) executed a two-day raid against Truk, thus opening the way to the complete neutralization of that formidable bastion. Following the Truk raid, Mitscher moved on to the Marianas and on 22-23 February administered to those islands their baptism of fire. The rear area had obviously become a forward area.

As the first step in recognition of the approaching threat to the Marianas, Carolines, and Palaus, the Japanese high command reorganized the command structure in the Central Pacific. For the first two years of the war, the 4th Fleet had commanded all Japanese garrisons in the mandates and was itself directly responsible to the Combined Fleet. By March of 1944 the 4th Fleet had lost effective control of its remaining garrisons in the Marshalls and had been further weakened by Mitscher’s February raids on Truk and the Marianas, On 10 March 1944 a new headquarters was placed between the Combined Fleet and the 4th Fleet and given control of all Navy and Army forces in the mandates and in the Bonin Islands to the north. This was the Central Pacific Area Fleet, commanded by Vice Admiral Chuichi Nagumo, who had led the attack on Pearl Harbor. Thereafter, 4th Fleet control was confined to naval garrisons in Truk and the eastern Carolines, though it was theoretically exercised over the lost garrisons in the Marshalls.

Naval garrisons in the Marianas, Bonins, and western Carolines (Palau Sector) fell under the direct control of the Central Pacific Area Fleet. Also, this new headquarters, in theory at least, commanded all Army forces in the mandates and the Bonins, commanding through Headquarters, 31st Army, In fact, Army troops remained practically independent of the naval command, and to all intents and purposes 31st Army had exclusive tactical and administrative control over all Army personnel in the area. Speaking of the Central Pacific Area Fleet headquarters, a Japanese naval commander captured on Saipan said, “It is a purely administrative command and has no tactical significance…. in actual fact it never got beyond the stage of paper organization.

Failure to establish clear-cut command relationships between the Army and Navy was characteristic of Japanese military organization in the Central Pacific, In the Marianas, as elsewhere, it had serious consequences. From the very beginning there was friction between Army and Navy from the highest headquarters to the lowest ranks. In early March, about the time the Central Pacific Area Fleet was officially placed in over-all command, a furious administrative squabble arose between that headquarters and the 31st Army, the latter objecting to being subordinate to the Navy.

The final decision, reached on 15 March, took the form of a compromise between the two headquarters. It was agreed that the command of each separate island was to rest with the senior Army or Navy officer present. It was also agreed orally between the Commander in Chief, Central Pacific Area Fleet, and the Commanding General, 31st Army, that neither would assume complete responsibility, thus apparently leaving the area without a supreme command.

The failure to carry out the principle of a unified command was to prove seriously detrimental to the efficiency of future Japanese operations in the Central Pacific. The resultant confusion was further compounded because of the high degree of mutual interdependence that necessarily existed between Army and Navy units. For example, all of the air strength in the Central Pacific was naval, under command of the Central Pacific Area Fleet. However, many of the air installations were serviced by Army units. Similarly, although the Army was only partially dependent on the Navy for surface transport, Army convoys had to be escorted by Navy ships.

More significant than the administrative changes reorganizing the structure of command in the Central Pacific was the rapid acceleration of troop movements into the area following the fall of the Marshalls and the strike against Truk. By May 1944 the Japanese had five divisions, six independent brigades, and five independent regiments in the 31st Army area, supported by innumerable smaller units ranging in size from tank and antiaircraft artillery regiments down to independent machine cannon companies. About one third of the Army personnel in the Central Pacific was concentrated in the Marianas, including two divisions, two independent brigades, and two independent regiments.16 The two divisions sent to the Marianas were the 29th and the 43rd. The 29th was transferred from Manchuria to Saipan in February, later moving to Guam. The 43rd, organized in June 1943, moved from Japan to Saipan in late May 1944.

In addition, the Japanese organized sundry independent Army units for service in the Central Pacific Area. Units of battalion size and smaller were detached from their parent divisions and reorganized into eight expeditionary units, three of which were sent to the Marianas. The 1st Expeditionary Unit, consisting of four infantry battalions and two artillery battalions, was allocated to Saipan; the 5th Expeditionary Unit, of two infantry battalions and one artillery battalion, was moved to Pagan; and the 6th Expeditionary Unit, made up of six infantry battalions and two artillery battalions, was sent to Guam.

In May, after most of the expeditionary units had reached their destinations, Army Section, Imperial General Headquarters, ordered a reorganization of the expeditionary units into independent mixed brigades and independent mixed regiments. In the Marianas the 1st Expeditionary Unit (Saipan) became the 47th Independent Mixed Brigade, the 5th Expeditionary Unit (Pagan) became the 9th Independent Mixed Regiment, and the 6th Expeditionary Unit (Guam) was divided into the 48th Independent Mixed Brigade and the 10th Independent Mixed Regiment.

The Navy, too, began to increase its strength in the Marianas shortly after the fall of the Marshalls. The 55th and 65th Naval Guard Forces were established on Saipan and Tinian, respectively, and several antiaircraft artillery units were also dispatched to the area. A large variety of small administrative offices were established to handle the greatly increased volume of supplies and troop movements, and labor units for construction work and loading and unloading ships were added to the garrisons. Finally, air strength was poured into the Marianas during the period as local airfields were developed and war came closer, so that many naval airmen and maintenance men swelled the number of naval air personnel in the islands.

This accelerated movement of troops into the Marianas was not allowed to go uncontested by the ever-advancing American forces. From the very beginning of the Japanese attempt to reinforce the islands, submarines of the U.S. Pacific Fleet began to take their toll. American submariners played an important role in the capture of the Marianas; in fact it can be said that the American campaign actually began in February with the first submarine attacks on Japanese troop convoys bound for Saipan.

The first of the major Japanese troop movements involved the transfer of the 29th Division, which left Ujina, Japan, on 26 February aboard three troop transports. Late in the afternoon of the 29th the convoy was attacked by American submarines. One transport (Sakito Maru), laden with 3,080 troops was torpedoed and sunk. Only 1,688 were rescued. The survivors (members of the 18th Infantry Regiment) landed on Saipan with almost no equipment. According to one report, “All their weapons were lost except seven rifles, one grenade thrower, two light machine guns and 150 bayonets.”

The next large convoy to sail for the Marianas left Yokohama on 12 March and carried the 1st, 5th, and 6th Expeditionary Units. The convoy was attacked by American submarines and, although no Army troops were lost, a naval transport (Kokuyo Maru) carrying 1,029 reinforcements for the 54th Naval Guard Force on Guam was torpedoed and sunk.

In April two more convoys left Japan for the Marianas, and although the first was attacked by submarines and two of its ships sunk, all troops were rescued and put ashore safely on Saipan.[N4-22CIC] The last two convoys to reach the Marianas, however, arrived with the units seriously depleted and without their equipment. The first of these departed Yokohama on 5 May and reached Saipan on the 14th. None of the troops carried aboard were originally intended for the Marianas, but when two of the transports were torpedoed the survivors were landed on Saipan. Fifteen hundred of these, members of the 9th Expeditionary Unit bound for Yap, remained on Saipan until the American invasion. About six hundred were reorganized as a battalion of the 47th Independent Mixed Brigade, but the rest remained on Saipan as ill-equipped stragglers. Other survivors of the same ill-fated convoy belonged to the 15th Infantry Regiment, destined for the Palaus. They formed another straggler group on Saipan, where the American invasion caught them still awaiting transportation to the Palaus.

[N4-22CIC: CINCPAC-CINCPOA Trans 10931, a file of orders and tables showing troop movements and locations of units in the Central Pacific Area. The Joint Army-Navy Assessment Committee, Japanese Naval and Merchant Shipping Losses During World War II By All Causes (Washington, 1947) (hereafter cited as JANAC, Japanese Shipping Losses).]

The last major troop movement to the Marianas, and certainly one of the most significant, was the transfer of the 43rd Division from Japan to Saipan. Arriving only a few weeks before the American invasion, the division was to play a leading role in the defense of the island, and its commander, Lieutenant General Yoshitsugu Saito, would assume effective control of the Saipan defenses. For transportation purposes the division was divided into two echelons, the first of which made its way safely through to Saipan sometime in May. The second was not so fortunate. On 30 May a convoy of seven transports carrying more than 7,000 troops of the 43rd Division sailed from Tateyama and headed south. It was subjected to almost continuous submarine attack, and within three days five of the seven transports were sunk. The two remaining vessels picked up the survivors and steamed the rest of the way to Saipan.

About 80 percent of the troops of this convoy were saved and landed on Saipan, but they arrived as hapless survivors without weapons or equipment. The 118th Infantry Regiment lost about 850 men, and the survivors had virtually no resemblance to the organized fighting team that had left Japan. So little time was left before the American invasion that the regiment could not be reorganized and re-equipped sufficiently to raise its combat efficiency much above nil.

Altogether, from January to early June 1944, the Japanese dispatched about 45,000 Army troops to the Marianas. Of these about 40,000 were allotted to Saipan, Guam, and Tinian; the remainder to Pagan and Rota. At least 12,000 of these troops were aboard torpedoed vessels, and about 3,600 died as a result of the sinkings. While many of the survivors were successfully reorganized, rehabilitated, and re-equipped, about half, perhaps four or five thousand, became stragglers on Saipan, equipped and armed only with their resolution to die for the Emperor. Thus, well before the initial American strikes against Saipan from surface ships and aircraft, U.S. submarines had seriously disrupted Japan’s major effort to reinforce the Marianas against the imminent threat of invasion.

Military Construction in 1944

Along with their hasty and not altogether successful effort to enlarge the garrisons of the Marianas, the Japanese in early 1944 undertook an ambitious program of building up the islands’ fortifications and defenses. First priority was assigned to airfield construction. At the beginning of the war Saipan had an airfield (Aslito) and a seaplane base and Tinian had an airfield. All three appear to have been operational. The situation remained unchanged as late as mid-1943, when the Imperial Navy commenced a new program of airfield construction in the Marianas and Carolines, planning to increase the operational airfields from one to two on Saipan and from one to three on Tinian. In addition, the program called for two new fields on Guam, and one each on Pagan and Rota, where none had existed before.

By February 1944 Aslito airfield was being enlarged, and two more fields on Saipan were under construction. The Saipan seaplane base, constructed in 1935, was in full operation. On Tinian, the Ushi Point field was in operation, and another field was under construction. On Guam, Sumay airfield was almost completed, and three other fields and a seaplane base were under way or in the planning stage.

In March plans were developed for an even more rapid build-up of airfield facilities. Combined Fleet and Central Pacific Area Fleet each issued orders outlining an ambitious policy of airfield construction. The Combined Fleet order provided that three independent complexes of bases were to be rapidly completed in the Marianas-Truk area and in the Palau-Yap area.

The Central Pacific Area Fleet order defined construction policy as: “To build rapidly a large number of bases so as to make possible the immediate development of great aerial strength.” In the Marianas, the bases were first to be built on a rough and ready basis and then gradually brought to a finished state as conditions permitted. Each field was to be integrated into the land defenses; machine gun positions for plane-mounted guns were to be built near plane shelters, and air force personnel were to be organized for ground combat.

These plans were intended to result in fourteen airfields and two seaplane bases: Saipan, three airfields and a seaplane base; Tinian, four airfields; Guam, four airfields and a seaplane base; Pagan, two airfields; Rota, one airfield. Each airfield was to be capable of handling forty-eight planes, except for the Marpi Point field on Saipan, which would handle twenty-four, and the Charan Kanoa strip, also on Saipan, which was for emergency use only. The entire network of bases would be sufficient for six hundred planes of various types.

Although the Combined Fleet order specified that normal air facilities were to be completed by April, the estimate proved much too optimistic. When the American landing forces came upon the scene in June, much of the construction was still unfinished. On Saipan, Aslito airfield was, of course, operational, as was the Charan Kanoa emergency airstrip, but the Marpi Point field was still unfinished and nonoperational.

Guam had two operational strips and two still unfinished. Tinian, the air center of the Marianas, had three fields finished and one to go. Rota and Pagan each boasted a surfaced runway. Altogether, of the fourteen fields planned, nine had been completed and were capable of handling at the most a total of four hundred planes rather than the originally planned six hundred.

While the Imperial Navy was hurriedly attempting to build up air strength in the Marianas, 31st Army was making similar plans for ground fortifications. The precis of the 31st Army defense plan gives a general outline of the Army fortification program: While deploying the troops the defensive constructions must be strengthened and general preparations for the annihilation of the enemy landing on the beaches must be completed, including the protection of our air bases. The field positions must be completed within one month after the arrival of troops and within three months the positions must be strengthened by constructing permanent fortifications in the most important points.

The hope that this schedule could reach completion before the American landings, if ever seriously held, proved a vain one. The 43rd Division did not arrive on Saipan until early June, a matter of days before the invasion. The 7th and 16th Independent Engineer Regiments, which were responsible for most of the “permanent fortifications,” arrived in April, only two months before the invasion. The job, even under ideal conditions, would have been difficult, and conditions were far from ideal. Troops arrived in a haphazard fashion, often depicted in number and missing their arms and equipment. Moreover, American submarine warfare accounted for the loss of more than mere manpower. Essential building matériel went to the bottom of the sea along with the men.

A report, dated 31 May, from the chief of staff of the 31st Army gives a clear survey of the difficulties facing the Japanese in their hurried effort to construct adequate fortifications on Saipan and the other islands in the Marianas: We cannot strengthen the fortifications appreciably now unless we can get materials suitable for permanent construction. Specifically, unless the units are supplied with cement, steel reinforcements for cement, barbed wire, lumber, etc., which cannot be obtained in these islands, no matter how many soldiers there are they can do nothing in regard to fortifications but sit around with their arms folded, and the situation is unbearable.

I would like this matter of supply of construction materials dealt with immediately. Japanese Doctrine for Island Defense The failure on the part of the Japanese to meet their time schedule in reinforcing the Marianas’ physical defenses had an important effect on the tactical doctrine to which they adhered throughout the campaign. It was one of the factors that compelled them to rely more heavily on beach defenses than would logically have been called for by the size and physiographic features of these particular islands.

In the Gilberts and Marshalls, Japanese defensive doctrine stressed defense at the beaches. At Tarawa, Makin, Kwajalein, and Eniwetok the fortifications were concentrated on a thin line along the shores with little defense in depth. By concentrating the majority of their positions on the beaches proper, and with the aid of fringing coral reefs and offshore obstacles, the Japanese hoped to annihilate the Americans before they could gain a foothold ashore. If the U.S. troops did succeed in establishing a beachhead, doctrine called for a counterattack to push them back into the sea.

This doctrine of island defense was based to a great extent on purely geographical and terrain considerations. The Gilberts and Marshalls are composed of widely scattered coral atolls, each in turn composed of many small islets. Although the Japanese usually selected the larger of the islets for their bases, these still had very little area and were generally elongated in shape. Defense in depth was impractical not only because there was little depth to defend but also because what little there was usually contained an airstrip that had to be left free of obstacles if it was to land and dispatch planes. Moreover, the flat terrain of the atolls provided no natural features such as hills and caves that could be exploited to set up an adequate defense in depth.

The Marianas are much different. They are volcanic islands, not coral atolls. They are generally much larger in size, have considerable elevation, and the terrain is rugged and mountainous, providing favorable opportunities for defense in depth. Yet, in spite of this the Japanese continued to place great emphasis on defending the shore lines of Saipan, Tinian, and Guam to the consequent neglect of fortifications and gun positions in the interior.

The continued reliance on beach defenses is illustrated in the defense plan for Saipan drawn up by the 1st Expeditionary Unit: Tactical Command Doctrine

  1. Objective

It is expected that the enemy will be destroyed on the beaches through a policy of tactical command based on aggressiveness, determination, and initiative…When the enemy elements are attempting to land: The main fire-power will be directed at the enemy landing forces prior to their arrival on the beach. Taking advantage of the confusion, the enemy will be rapidly destroyed by counter attacks, mounted from all sectors wherever the opportunity presents itself.

Should the enemy succeed in gaining a foothold on the beach, intense fire will be concentrated and determined counter-attacks launched with the aid of reserves and tanks. Although the advantages of surprise will be lost, the enemy landing forces can be dealt with by further attacks after night fall.

Later in the war and under conditions similar to those obtaining in the Marianas, the Japanese abandoned or modified their earlier doctrine and concentrated heavily on defenses in depth. When the U.S. Marines landed on Iwo Jima, they found a well-prepared network of defenses in depth as well as fortifications commanding the shore line.35 At Okinawa the landing beaches on the west coast were left practically undefended, and the main Japanese forces had retired before the invasion to the southern part of the island, where they holed in along the Shuri line with its elaborate system of caves and underground installations.

Why, then, did the Japanese as late as June 1944 cling to the older concept, which though valid enough for atoll warfare was clearly not so suitable for the type of fighting that would inevitably develop in the mountainous terrain of the Marianas? The failure to adjust tactical doctrine to changing conditions of terrain can probably be attributed in part to the highly aggressive spirit of the Japanese military mind. Generally, the Japanese preferred to sally forth sword in hand against the enemy rather than bide their time in prepared positions.

As noted in the U.S. Army handbook on the subject of Japanese tactics: No matter what the situation, a Japanese commander’s first reaction to it is to act aggressively to maintain the traditions of his army….Even when the Japanese commander assumes the defensive, he will, so far as possible, carry out that defense by using the most aggressive tactics that the situation permits.

In the case of the Marianas, there was an even more compelling consideration that forced the Japanese to rely most heavily on their beach defenses. There was simply not enough time to complete the fortification program. The 31st Army program for defense had made provision for falling back upon prepared “strategic inland positions” in the event that an enemy landing force was not thrust back into the sea. That plan reads, in part: First priority in construction will be given to improvised positions designed to frustrate enemy landings on the beaches…and to temporary protective measures designed to minimize our losses in personnel and materials. Later, these constructions will be rapidly supplemented and strengthened by extending the positions in depth, converting actually the island into an invulnerable fortress. The speed-up of the U.S. invasion plans, coupled with the loss of valuable building materials to the U.S. submarines, made the fulfillment of the second part of this plan impossible.

In line with the doctrine of defense at the beaches, the 31st Army planned to cover all segments of the shore line “where the enemy could land without difficulty” with independent strongpoints several hundred meters or one kilometer apart. Each strongpoint would be manned by an infantry company or a heavy weapons platoon. Patterns of fire were to be arranged so that each strongpoint could provide flanking fire on its neighbors’ fronts, and areas not otherwise provided for were to be covered by machine guns and mortars. In this manner, whole companies would be deployed along the beaches.

Behind the coastal positions, at a distance depending on terrain, was to be constructed a second line that would cover any partial collapse of the coastal positions and serve as a starting point for counterattacks by reserve units. The second line was to be organized on the same principle as the coastal positions. The 31st Army plan emphasized the construction of dummy positions between strongpoints and between the coastal positions and the second line. These positions were to deceive the enemy and divert his fire. They were to be especially thick on those stretches of the shore line where enemy landings were less likely because of natural obstacles, and where only small forces or lookouts might be stationed.

Since the artillery was to fire anti-boat missions, many pieces were to be emplaced in the coastal positions and the second lines, as well as farther to the rear. Antiaircraft artillery was to be located in positions where it could lend support to ground actions as well as fulfill its primary function. Finally, the plan made rather vague provision for final strongpoints to the rear of the second lines, providing that “if time allows the rear positions…must be strongly built and also completely equipped for the counter-attack.” Actually, time did not allow the Japanese to provide for such positions.

Enemy Troop Strength and Dispositions on Saipan

Command of all Army troops in the Marianas rested with the 31st Army under Lieutenant General Hideyoshi Obata, who had headquarters on Saipan. When the American forces landed on 15 June, Obata was in the Palau Islands and consequently exercised no tactical command during the campaign. His headquarters on Saipan, consisting of about 1,100 officers and men, was largely administrative in function and had little tactical significance. The largest single Army unit on the island at the time of the landing was the 43rd Division, commanded by General Saito. It was Saito who actually exercised tactical command until his death a few days before the close of the battle. The division consisted of three infantry regiments, the 118th, 135th, and 136th, plus a signal company, a transport company, an ordnance company, a field hospital, and an “intendance duty unit” responsible for quartermaster and finance functions. Altogether, the division numbered about 12,939 officers and men.

Next in size among Army units was the 47th Independent Mixed Brigade, commanded by Colonel Yoshiro Oka. The brigade had four organic battalions, but one, the 315th, was on Pagan. The three others, the 316th, 317th, and 318th Independent Infantry Battalions, were on Saipan. An engineer unit and an artillery unit, the latter consisting of one battery of eight field guns and two batteries of seven howitzers each, also belonged to the brigade. Total strength of the brigade on Saipan at the time of the landing was about 2,600.

To this nucleus was attached a host of smaller units including many that had originally been destined for other islands but had been stranded on Saipan as a result of shipping damage inflicted by American submarines. The most important of these were the 3rd Independent Mountain Artillery Regiment consisting of two battalions, each with twelve 75-mm. mountain guns; the 16th Shipping Engineers Regiment; the 7th Independent Engineer Regiment; the 9th Tank Regiment with thirty-six medium arid twelve light tanks; and the 25th Antiaircraft Artillery Regiment. Total strength of Japanese Army troops on Saipan on the eve of the American invasion was about 25,469.

To this number must be added about 6,160 naval personnel. Among the naval units present on the island were the Headquarters, Central Pacific Area Fleet, under Admiral Nagumo and the Headquarters, 5th Base Force, under Rear Admiral Takahisa Tsujimura. The latter unit had been in command of the Marianas since before Pearl Harbor, but with the arrival of Army troops in 1944 it assumed a less important role in the defense of Saipan, though it continued to command naval shore forces and surface units within the Marianas. The largest single element of the naval forces was the 55th Naval Guard Force, about 2,000 officers and men, which was chiefly responsible for manning coast defense guns. The only other large naval unit was the Yokosuka 1st Special Naval Landing Force, consisting mainly of a headquarters, three rifle companies, a machine gun platoon, and a gun section.

For purposes of ground defense, Saipan was divided into four parts. The first was the Northern Sector, that portion of the island north of a line beginning at Flores Point on the west coast and extending to the east coast in a southeasterly direction. The most important troop unit located here was the 135th Infantry Regiment, less one battalion.

The second division was the Navy Sector, which was bounded on the north by the Northern Sector, on the west by the shore line from Flores Point to a point just below Garapan, thence on the south by a line to a point just southwest of Mount Tapotchau’s summit, and on the east by a line from this point up the axis of the island to the center of the Northern Sector’s boundary. The 5th Base Force controlled the Navy Sector, but the beach defenses were actually manned by a battalion of the 136th Infantry Regiment.

The Central Sector lay directly south of the Navy Sector. Its shore line extended from a point below Garapan to Afetna Point. Inland, to the east, it was bounded by an extension of the Navy Sector’s boundary down the central axis of the island to a point directly east of Afetna Point. Here the boundary turned west to Afetna, enclosing the sector from the south. Responsibility for the defense of the Central Sector rested with the commanding officer of the 136th Infantry Regiment. His regiment was reduced to the 2nd Battalion reinforced by one company of the 3rd, since the 1st Battalion was assigned to the Navy Sector and the remaining two companies of the 3rd Battalion were in 43rd Division reserve.

The fourth defense area was the Southern Sector, whose shore line extended along the west coast from Afetna Point, around the entire south coast, and up the east coast beyond Kagman Peninsula to the boundary line of the Northern Sector. This sector, commanded by the commanding officer of the 47th Independent Mixed Brigade, encompassed about half of the island, and probably contained the same proportion of the island’s troops. Here were concentrated all the reserve troops as well as the forty-eight tanks, the main artillery batteries, and the antiaircraft units clustered around Aslito airfield.

Within each of these sectors, the disposition of troops was generally in accordance with the doctrine of concentrating on defeating the enemy at the shore line. For example, a 136th Infantry Regiment operations order dated 25 May makes the following disposition of the regiment’s forces in the Central Sector, which included most of the shore line upon which the 2nd Marine Division was to make its landing:

Front Line (Beaches) 4th Company; 5th Company; 6th Company (less 1/3);8th Company (less 1/3); 2nd Infantry Gun Company; Sector Reserves: 1/3 6th Company; 1/3 8th Company.

The commitment to beach defense of such a large portion of troops of the 136th Regiment with the consequent neglect of reserve power was entirely consistent with prevailing Japanese tactical doctrine.

Reserves for the island were located in the area just north of Magicienne Bay and included two companies of the 3rd Battalion, 136th Infantry Regiment, with two companies of the 3rd Battalion, 9th Independent Mixed Brigade, attached. The 16th Shipping Engineers Regiment (less detachments) and the main body of the 9th Tank Regiment were also in this area.

In case of landings in the Garapan-Tanapag sector, the tanks were to assemble in the mountains two miles east of Garapan and there prepare to counterattack. If landings took place in the Magicienne Bay or Charan Kanoa area, the tanks were to assemble about one-half mile north of Aslito airfield’s west end and prepare for operations against either area, or both.

Most of the Army mobile artillery, organized as the 3rd Independent Mountain Artillery Regiment and the 47th Independent Mixed Brigade Artillery Unit, was located along the ridge overlooking the Charan Kanoa beaches to the north and south of Mount Fina Susu. From these positions the guns were to fire missions on the east and west coasts. Japanese artillery preparations on Saipan were careful and elaborate, preliminary sightings having been made on important points such as breaks in the reefs. However, these preliminary sightings seem to have been conducted independently by separate pieces of batteries, and no common survey control was established. As a result, the Japanese were unable to mass fires by tying in batteries or battalions, and the Japanese artillery dissipated itself in local un-coordinated missions. Another defect in Japanese artillery on Saipan arose from the lack of prime movers. All of the field artillery was mobile, but there were neither horses nor trucks on the island and the pieces therefore had to be moved by hand, if at all.

In addition to the Army’s mobile artillery, Saipan could boast an elaborate system of naval coast defense and dual-purpose batteries. The Japanese Navy’s scheme of artillery fire was intended to cover almost every square inch of both Saipan and neighboring Tinian, large sweeps of the adjacent sea, and nearly all of the air overhead. According to the Navy’s plan, coast defense guns (12, 14, and 15-cm’s.) were to be lined up along the west coast of Saipan from Agingan Point to Tanapag in a series of seven batteries covering the entire approach from the west with interlocking sectors of fire. A second concentration of Navy coast defense guns, in five batteries, was to be emplaced on Nafutan Point and the shores of Magicienne Bay, covering the sea approaches to Saipan from the south and southeast. A battery of 4-cm. guns on Marpi Point emplaced to fire to the north would complete the picture. Two batteries of 12-cm. dual-purpose guns were to cover the Aslito airfield; two more of the same size were to cover the center of island; an 8-cm. battery, the Tanapag area; and a battery of 12-cm’s., Marpi Point.47 Not all of these emplacements were completed at the time of the American landings, but most of the Navy’s coast and aerial defense program was in working order by mid-June of 1944.

Army antiaircraft batteries on Saipan were spread from Mutcho Point to Aslito airfield, mostly along the ridges and hills overlooking the west beaches. The air over the entire southern portion of the island was covered by the guns of the 25th Antiaircraft Artillery Regiment’s 1st, 2nd, and 6th Batteries, The central portion of Saipan was covered by the 43rd Independent Antiaircraft Company and the 45th Field Machine Cannon Company. In general, however, a shortage of ammunition handicapped the antiaircraft artillery. Maximum allowance for antiaircraft weapons on Saipan was three units of fire, but in fact the 25th Antiaircraft Artillery Regiment and its two attached companies probably did not have as much as one unit of fire (4,800 rounds) on hand. Antiaircraft ammunition was in short supply not only in the Marianas but in all other 31st Army areas as well.

Japanese Expectations

Immediately after the February strike against Truk by Marc Mitscher’s fast carrier task force, the Japanese reached a state of near panic in their apprehensions of an imminent American attack against the Marianas. Preparations for moving the 31st Army headquarters to the Marianas had already begun when U.S. forces seized the Marshalls and Eniwetok, and the headquarters staff was scheduled to leave Tokyo for Saipan when Mitscher’s follow up strike on the Marianas took the Japanese by surprise. This strike occurred on 22-23 February, and on the following day General Tojo himself suddenly ordered the staff to depart at once. On 3 March 31st Army issued an emergency plan for the defense of the archipelago, which at that time was virtually defenseless.

On 4 March civil government was abolished on Guam, and shortly afterward martial law was established throughout the islands. In early March the Japanese actually feared an invasion of the Marianas by the end of the month, as indicated by the following excerpt from the 31st Army staff diary: “The central command was very much concerned over the enemy attacks. After the enemy striking force hit the Marianas Sector on the 22nd and 23rd of February, thereby revealing our lack of defenses, they were afraid of an enemy invasion in March.” 50 The sinking of Sakito Maru, which was carrying the 18th Infantry Regiment to Saipan, only increased the alarm. For a while 31st Army headquarters planned to divert the 8th Expeditionary Unit, bound for Truk, to the Marianas, but this plan was not carried through. By late March the apprehension over an immediate invasion had somewhat abated, and Army troops were being used to build airfields.

Then at the end of March Mitscher raided the Palaus, and on 22 April MacArthur captured Hollandia in Netherlands New Guinea. The Japanese pulse once again beat faster. The Japanese Navy and Army high command differed somewhat in their estimates of the situation. The raid on the Palaus, the capture of Hollandia, and, finally, the operations around Biak in May led the naval high command to anticipate that the next American attack would come in the Palaus or Carolines rather than as far north as the Marianas. On 3 May Admiral Soeumu Toyoda assumed command of the Combined Fleet and received a directive from Imperial GHQ outlining the A Operation (also known as the A-GO Operation), which called for the surface striking force and land-based air units to be prepared for a decisive engagement with the U.S. fleet by the end of May, presumably in the Palau-Carolines area.

Army officers in Imperial GHQ at the same time seem to have taken more seriously the possibility of an early invasion of the Marianas. On the assumption that the main objective would be the Philippines, they concluded that the Americans might advance along the north coast of New Guinea or into the Carolines and Marianas. It was also estimated that “there is a great possibility that both of the operations will be commenced simultaneously.”

Whatever may have been their belief in regard to an ultimate invasion of the Marianas, it is clear that the Japanese did not expect a full-scale attack against Saipan by mid-June. As late as 8 June, 31st Army headquarters issued an order for the transfer of troops from Saipan to other islands in the archipelago, which, had it been carried out, would have reduced the Saipan garrison by 2,500 men. An even more striking indication that the Japanese were caught off guard by the rapidity of the American approach comes from an order issued by Admiral Nagumo on 14 June, the day before the first American troops hit the beaches on Saipan, that reads, in part, “It is a certainty that he [the enemy] will land in the Marianas Group either this month or next.”

The conclusion, then, is inescapable that the Japanese were simply not prepared for the American landings on Saipan when they came. In the short time intervening between the American capture of the Marshalls and the invasion of Saipan, the Japanese found it physically impossible with the means at hand to build up the defenses of the Marianas to a point where they might successfully resist a landing by U.S. forces. In their race against time the Japanese were further handicapped by the deadly efficiency of American submarines, which by now had extended the radius of their operations well to the west of the Marianas perimeter. Finally, U.S. forces achieved a measure of surprise by attacking Saipan before they were expected, thus adding the element of shock to the already preponderant strength they were able to bring to bear against this vital point in Japan’s inner defensive perimeter.

Prepared or not, the Japanese forces on Saipan were by no means feeble. The more than 30,000 troops on the island were backed by forty-eight tanks and an elaborate, if incomplete, network of artillery positions and reinforced by their determination to die if necessary for the Emperor. They could be expected to put up a fierce battle.

SOURCE: Campaign in the Marianas; BY: Philip A. Crowl (United States Army Center of Military History)

World War Two: Saipan (2-5); Allied Invasion June 1943

World War Two: Marianas (2-3) Planning the Invasion-Saipan &Guam


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