How Japan Fortified The Mandated Islands

Between world wars, any American who thought about the possibility of war with Japan wondered if the Japanese were fortifying the Mandated Islands. The relative strengths of the United States and Japan in any future war hinged on this question, for the United States would have to defend the Philippines, and the Philippines could be cut off from the United States by Japanese bases in the Mandates. The Mandated Islands surrounded Guam and menaced Wake, thus straddling America’s thin lifeline across the Pacific. If Japan built bases in the strategically located islands, she could sever the lifeline and extend her empire in the Far East with little fear of immediate American interference. But in spite of the advantages of bases in the Mandates, Japan was bound by international agreements not to build them. Americans could only hope that Japan would keep her agreements.

The Mandates consisted of three groups, the Marianas, Carolines, and Marshalls, which Japan seized from Germany in 1914. The League of Nations later awarded Japan a mandate for the groups, on condition that Japan promise not to fortify or establish bases in the mandated territory. Japan made the same promise to the United States in a 1922 treaty. Neither the mandate terms nor the treaty with the United States provided any means of ensuring that Japan would not fortify the Mandates. The League could not even inspect the islands without Japanese permission. When Japan announced her withdrawal from the League in March, 1933, she took the Mandates with her, and the weak international organization could do nothing but acquiesce.

After 1933, Japan continued to submit reports on the Mandates to the League. No trace of forbidden fortifications appeared in the reports, but Japan would not allow League representatives to inspect the islands. Japan also excluded foreign vessels from Mandates ports, seldom permitted foreign travellers in the islands, and severely restricted the movements of the few foreigners she did admit. Such security measures gave rise to rumors that Japan was secretly fortifying the islands, rumors which persisted in spite of frequent Japanese denials and in spite of an absence, at least in the public press, of definite evidence.

On several occasions the United States tried to lift the clouds of secrecy and suspicion that enveloped the Mandates. In 1936 the State Department invited two Japanese ships to Alaskan harbors normally closed to foreign vessels, on condition that Japan invite an American ship to visit closed ports in the Mandates. Japan refused. In January, 1940, when a Japanese fishing vessel was wrecked on Guam, the United States Navy offered to put the shipwrecked crew ashore at Saipan, the nearest Mandates port. Japan refused to expose Saipan even to this casual visit, and the survivors had to be put aboard a Japanese ship at sea. Such incidents made it appear more than ever that Japan was concealing fortifications, despite her denials, and increased rather than alleviated the suspicions of the United States.

American suspicions found public expression in highly speculative journalistic accounts of Japanese fortifications in the Mandates. Despite the lack of positive evidence, the speculations gave rise to a myth that Japan spent the two decades between wars secretly transforming the Mandates into impregnable island bastions. The outbreak of World War II in the Pacific showed that the myth had some foundation in fact, for early Japanese operations were often supported from the Mandates, and Admiral Halsey’s task force raid in the Marshalls in early 1942 was resisted by island air bases, bristling with coast defense guns, that must have been started before the war. But there was still no evidence to show when Japan started her bases, which islands were fortified, and how far the bases were developed before the war.

The Tokyo War Crimes Trials, held just after the war, unearthed many facts on the problem, but they were used to prove prosecution charges on the one hand and to support the innocence of the defendants on the other, leaving little room for an objective appraisal of what actually happened. Since the trials, particularly in the last three years, much new information on Japan’s secret construction in the Mandates has come to light, some from recent writings of former Japanese naval officers, and more from voluminous Japanese naval records not introduced at the trials. The new material allows a more complete reconstruction than ever before of the story of Japan’s Mandates fortifications before Pearl Harbor.

Japan made no improvements of permanent military significance in the Mandates for twenty years after she took them from Germany in 1914. The Imperial Navy’s World War I occupation troops, of course, required military installations, but the troops were withdrawn in the early 1920’s, and the installations were discontinued in conformance with treaty restrictions. Japan first constructed permanent bases in 1934, a year after withdrawing from the League of Nations. From 1934 through 1939 the Imperial Navy and the South Seas Bureau, civil government of the islands, cooperated in a limited construction program, aimed at creating first three, and later four, key bases. The 1934-1939 program established no bases in the Marshalls, sent no garrisons to any of the islands, and attempted to avoid outright and obvious treaty violations.

The key islands were Saipan, the Palaus, Truk, and later Ponape. First major projects were Aslito airfield at Saipan and a seaplane ramp in the Palaus, both started in 1934. The Imperial Navy started a ramp at Saipan and an airfield at Truk in 1935, and airfields in the Palaus and Ponape in 1938 and 1939 respectively. The Japanese also began harbor improvements at Saipan and the Palaus, communications centers at Saipan and Truk, and oil storage facilities at all four key bases. The Japanese started airstrips at Tinian and Pagan and perhaps at other islands, but the 1934-1939 program concentrated on Saipan, the Palaus, Truk, and to a lesser degree, Ponape. These bases were far from formidable at the end of 1939. Of four airfields and two ramps underway, only the Saipan airfield and the ramps were completed, and there were no fortifications, guns, or troops to defend them.

The Japanese believed that the 1934-1939 program did not violate the treaty restrictions, which merely forbade “bases” and “fortifications” without defining the terms. The air, harbor, communications, and fuel facilities were, in Japanese eyes, improvements in the islands’ civilian economy and not “bases” or “fortifications” at all. Since the key islands were actually administrative and commercial centers that could use economic improvements, the Japanese argument had its merits. Even the fact that these very facilities were later used in World War II does not detract from their legality through 1939. Indeed, Japan might have made a good case for her adherence to international law had she opened the Mandates to foreign observers before the end of 1939. Her refusal to do so apparently stemmed from a general policy of strict military security and a reluctance to expose her interpretations of the treaties to hostile criticism.

In 1939 and 1940 the United States emerged as an increasingly stronger threat to Japan’s East-Asian empire. To bolster her island-studded flank against American intrusion, Japan decided to augment the limited 1934-1939 program with new construction that would increase and strengthen the Mandates bases.

In 1939 the Imperial Navy decided to expand construction to the Marshalls. Until then, the Japanese believed that the Marshalls, farther from Japan than the Marianas and Carolines and most exposed to American attack, would be too difficult to defend. However, improvements in warships and naval weapons, especially the advent of heavy land-based bombers, made bases in the Marshalls more desirable. The Imperial Navy sent a large team to survey the group in the summer of 1939, and soon after selected Kwajalein, Wotje, Maloelap, and Jaluit for development as new key bases. This decision had a profound effect on the subsequent course of World War II and led directly to Japan’s seizure of the Gilberts in 1941 and to the bloody beaches of Tarawa two years later.

_{Table 1. Airfields and Seaplane Ramps in the Mandated Islands, December 8, 1941}

Location	Type	Started	Completed
Marianas
Saipan	airfield	1934	1935
	seaplane ramp	1935	1935
Tinian	airstrip	1939-40	1941?
Pagan	emergency airstrip	1934?	1940?
Western Carolines
The Palaus	airfield	1938	1940
	seaplane ramp	1934	1936
Eastern Carolines
Truk	airfield	1935	1941
	airfield	1940	1941
	seaplane ramp	1940	1941
Ponape	airfield	1939	1941
	seaplane ramp	1940	1941
Marshalls
Kwajalein	airfield: 1 runway	1940	1942
	airfield: 3 runways	1940	1941
	seaplane ramp	1940	1941
Wotje	airfield: 2 runways	1940	1941
	seaplane ramp	1940	1941
Maloelap	airfield: 2 runways	1940	1941
Jaluit	seaplane ramp	1940	1941

^{Note: Construction listed in this table has been established to a high degree of certainty. Additional air installations may have been started on Saipan, Tinian, the Palaus, Wotje, and Majuro before December 8,1941. There is no evidence that any more than those so indicated above were completed by that time.}

The Japanese began building the four Marshalls bases in 1940, making a total of eight key bases underway. Garrison troops, with ships and planes, were sent to the Mandates for the first time. Construction, previously confined to installations that could be explained away as economic improvements, now came to include facilities of unequivocal military character. The old 1934-1939 program was not only enlarged in geographic area and re-oriented from quasi-military to overt military construction, but both old and new bases were rushed toward completion at an accelerated pace, and more was accomplished in the two years preceding Pearl Harbor than in all the years before.

Seventy per cent of the money for Mandates construction in 1940-1941 went into air facilities at the eight key bases. Four airfields and three seaplane ramps were started in the Marshalls, and one new airfield and two more ramps were begun in the Carolines (See Table 1). Only one airfield was completed during 1940, but the following year saw airfields and ramps finished at the rate of one a month. When the war started eight airfields and seven ramps were operational. Many still required additions, such as revetments and hangars, at the end of 1941, but their runways could be, and were, used by Japanese planes in the war’s first days.

After air installations, fuel facilities formed the most prominent category of construction in 1940-1941. The 1934-1939 program resulted in oil pumps and oil storage facilities at Saipan, the Palaus, Truk, and Ponape. These were improved in 1940, and new installations added at Jaluit. In February, 1941, the Japanese began additional fuel facilities, mostly for heavy oil, at all eight key bases plus Eniwetok. Fuel stored at these bases before Pearl Harbor was used by the Japanese fleet in the earliest operations of the war.

Harbor, navigation, and other installations introduced from 1934 through 1939 were also increased in the period of intensive construction that began in 1940. New construction included power generators, water storage, channel improvements, navigation beacons, a drydock, a hospital, wharves, and fifteen lookout stations.

At the Tokyo War Crimes Trials, the defense argued that there were no purely military installations in the Mandates until about a month before the Pearl Harbor attack, and that all construction before November, 1941, was merely commercial or cultural improvement. Japanese records, not introduced at the trials, tell a different story and show that outright military construction was carried on for almost two years before the war. The records show that the Japanese started building shore fortifications at all eight key bases in 1940, at a cost of 6,632,750 yen. By June, 1941, money was being allocated for maintenance of antiaircraft artillery, and in August 2,562,750 yen was set aside for additional gun positions. During the year 4,675,000 yen was devoted to submarine bases, to be completed by October 31, 1941, at Ponape, Kwajalein, and Truk. 246,300 yen went for oxygen-gas-producing stations, to service oxygen-powered torpedoes, at Ponape, Kwajalein, and Jaluit. Most of these projects were scheduled for completion in 1941, and like the airfields, may have lacked a few trimmings, but were ready for use by the end of the year.

A Japanese description of the 1940 project to install over six million yen worth of fortifications at the eight key bases was entitled “Lighthouse Construction.” This was, of course, an attempt at secrecy, for the text of the description makes no mention of towers, searchlights, or conventional lighthouse gear, and instead gives a carefully itemized list of command posts, ammunition depots, and barracks.

A similar attempt at security was a code system for island place names. Letters of the Roman alphabet stood for various islands, and each letter was usually preceded by the term “Tokyo Bay Area,” so that the uninitiated would conclude that an airfield in Tokyo Bay Area D meant an airfield near the shores of Tokyo Bay. Actually “Tokyo Bay Area” did not refer to the bay itself, but to the vast ocean areas south of it. Since Japanese authorities did not always use the code, and records often mentioned both the code letter and the actual place name side by side, the code is easily broken:

D—The Palaus

H—Pagan

O—Maloelap

E—Truk

J—Tinian

Q—Kwajalein

F—Saipan

L—Jaluit

R—Majuro

G—Ponape

M—Wotje

Though these attempts at security appear ineffectual, one security policy was both thorough and effective. In 1937 the Japanese Navy was allowed to divert money from one item in its appropriations to another. This policy must have been followed even earlier, for published budget figures give no hint that money was ever spent in the Mandates for military purposes. Money for Mandates construction actually came from at least four different funds within the Navy’s appropriations, including “Building and Repairs” and “Extraordinary Military Expenditures.” At least 121,000,000 yen was allocated to Mandates construction from various budget items in the year ending November 30, 1941. This was equivalent to $30,000,000, but, considering the comparative cheapness of labor and materials available to Japan, it could purchase far more in value.

The Mandates bases had no permanent garrisons capable of defending them until the final year before the war, though naval construction, communications, and weather personnel were in the islands from 1934 or earlier. In late 1940 permanent shore garrisons, as well as surface and air units, started moving to the Mandates from Japan. The garrisons were under Fourth Fleet, which established headquarters at Truk by the end of February, 1941. By the time of the Pearl Harbor attack, Fourth Fleet commanded numerous shore defense units, large numbers of construction personnel, four light cruisers, eight destroyers, a submarine force, an air flotilla, and innumerable transports, subchasers, minesweepers, minelayers, gunboats, and patrol vessels.

Table 2. Force under Headquarters Fourth Fleet in the Mandated Islands, December 8, 1941
Shore, Surface, and Air Units	Location	Earliest Known Date Present in Mandates
Headquarters Fourth Fleet	Truk	Feb 41
Departments:
Construction	Various islands	27 Feb 41
Fortifications
Intendance	Truk
Munitions	Various islands	Feb 41
Meteorological	Various islands	14 Mar 41
Harbormaster’s	Truk
Fleet Surface Force: CL Kashima, Flag
Crudiv 18: CL Tenryu, CL Tatsuta
Desron 6: CL Yubari
Desdiv 29: Oite, Hayate, Asanagi, Yunagi
Desdiv 30: Mutsuki, Kisaragi, Yayoi, Mochizuk
Minelayer Sqdrn 19: 3 minelayers and 2 maru
Various cargo vessels and tankers, a water carrier and a hospital ship
Fleet Submarine Force:
Subron 7: Tender Jingei
Subdiv 26: RO-60, RO-61, RO-62
Subdiv 27: RO-65, RO-66, RO-67
Subdiv 33: RO-63, RO-64, RO-68
Fleet Air Force:
Headquarters, 24th Air Flotilla	Roi
Chitose Air Unit	Roi	Oct 41
Yokohama Air Unit	Wotje	Oct 41
Tokyo Air Unit	Palaus	Oct 41
104th (former 4th) Air Repair Station	Palaus
105th  (former 5th) Air Repair Station	Saipan
Western Caroline Sector: Headquarters 3d Base Force
3d Base Force	Palaus	30 Dec 40
3d Defense Force	Palaus	19 Dec 40
3d Communication Unit	Palaus, Yap
16th (former 7th) Air Unit	Palaus	3 Dec 40
Conv Gunboat Division 4: 2 maru
Conv Minesweeper Division 13: 4 maru
Conv Subchasdiv 55: 4 maru
Fukuyama Maru
Eastern Caroline Sector: Headquarters 4th Base Force
4th Base Force	Truk
4th Defense Force	Truk, Ponape	20 Dec 40
4th Communication Unit	Truk, Ponape
17th Air Unit	Truk	8 Oct 41
Conv Gunboat Division 5: 3 maru
Conv Gun bat Division 6: 3 maru
Minesweeper Division 14: 4 maru
Conv Subchasdiv 56: 3 maru
Conv Subchasdiv 57: 3 maru
Conv Subchasdiv 58: 3 maru
Koei Maru
Marianas Sector: Headquarterws 5th (Special) Base Force
5th (Special) Base Force	Saipan
5th Defense Force	Saipan	13 Dec 40
5th Communication Unit	Saipan
18th (former 8th) Air Unit	Saipan	12 Dec 40
Conv Gunboat Division 7: 2 maru
Conv Minesweeper Division 15: 2 maru
Conv Subchasdiv 59: 3 maru
Conv Subchasdiv 60:3 maru
Shoei Maru
Marshalls Sector: Headquarters 6th Base Force
6th Base Force	Kwajalein
6th Defense Force	Four key bases	5 Mar 41
6th Communication Unit	Kwajalein	29 Mar 41
51st Guard Force	Jaluit	19 Oct 41
52d Guard Force	Maloelap	5 Nov 41
53d Guard Force	Wotje	1 Nov 41
19th Air Unit	Jaluit	20 Mar 41
Conv Gunboat Division 8: 3 maru	Jaluit	Jul 41
Conv Minesweeper Division 16: 4 maru	Kwajalein
Conv Subchasdiv 62: 3 maru	Kwajalein
Conv Subchasdiv 63: 3 maru	Kwajalein
Conv Subschasdiv 64: 3 maru	Maloelap
Conv Subchasdiv 65: 3 maru	Wotje
Hakkaisan Maru
Mitsushima Maru
Toyotsu Maru

 

Fourth Fleet was responsible for completing the last ten months of the 1940-1941 construction program. The Navy Ministry itself apparently handled the 1934-1939 program but delegated responsibility for the intensive construction of 1940 to the Yokosuka Naval District, which turned over the program to Fourth Fleet’s Construction Department in February, 1941. The department was composed mostly of Korean coolies, who were conscripted, unskilled, civilian laborers, ill-trained and unsuited for combat. A Japanese cadre, including naval officers and civil engineers in top positions, held the Construction Department together. Though the department had between 10,000 and 20,000 men working in the Mandates, the actual labor force in the islands was much greater, for natives and garrison troops were also extensively employed.

Fourth Fleet divided the Mandates into four sectors—Western Carolines, Eastern Carolines, Marianas, and Marshalls—each under a Base Force commanded by a Rear Admiral, with their respective headquarters at the Palaus, Truk, Saipan, and Kwajalein. (See Table 2.) Each sector commander, in addition to his Base Force, had a Defense Force, a Communications Unit, considerable surface elements, and a small air unit. In consideration of its large number of key bases, the Marshalls sector received a reinforcement of three Guard Forces. All sectors received their first shore garrisons in December, 1940, excepting the Marshalls, where troops first stepped ashore in early 1941.

Base Forces ranged in strength from 165 to 1,613, and those in the Mandates were roughly equivalent to a battalion. They were responsible for both sea and shore defense in their sectors, as well as administration, harbor control, communications, and supply. Defense Forces, varying from 250 to 510, and Guard Forces, ranging from 400 to 900 men, supplied gun crews and troops to repel assault landings. Fourth Fleet, at sea, ashore, and in the air, had roughly 20,000 naval and 24,000 civilian (mostly labor) personnel in September, 1942, the earliest date for which sound figures are available. In November, 1941, rations were allotted for 36,000 persons in the Mandates, and this figure is close to the true strength when the war began.

The Mandates garrisons were well provided for by the 1940-1941 construction program. At a cost of 7,000,000 yen, Fourth Fleet staff departments were supplied with offices, warehouses, ammunition storage, and barracks. In September, 1941, over 10,000,000 yen was allocated for Base Force and Defense Force installations on Saipan, the Palaus, Truk, Kwajalein, and Ponape.

Records of the Mandates garrisons show the purely military character of their mission and illustrate the extent of their cooperation with the Fourth Fleet Construction Department. A typical garrison unit was the 6th Defense Force, organized January 15, 1941 at Yokosuka, where it underwent intensive combat training. Advance parties left Yokosuka February 24, arrived in the Marshalls March 5, and by the 8th were established at Wotje, Kwajalein, Maloelap, and Jaluit—the four Marshalls key bases. The main body reached the islands in May, and the Defense Force reported its activities for the next six months, from June 1 to November 30, 1941, as follows:

1. Engaged in construction of gun positions on each island.

2. Guarded Kwajalein, Jaluit, Taroa (Maloelap), and Wotje; defended and guarded adjacent surface areas.

3. Engaged in military fatigues in preparation for the (China) Incident.

4. Engaged in rapid construction of defense installations.

5. Supervised harbors on each island.

6. Performed skilled construction. Diagnosed diseases.

7. Engaged in supply of ships, special lookout stations, and weather stations.

8. Engaged in conducting necessary movements and maintained liaison among the various atolls.

9. Cooperated in construction fatigues of the Fourth Fleet Construction Department.

10. In accordance with war-time security, conducted strict anti-air and anti-submarine patrols.

11. Conducted accelerated training for all types of warfare.

According to the report, all this was being done in “readiness” for participation in the China Incident, as Japan called her war on the Asiatic Continent. Both the facts of geography and the contents of the report make it clear that these troops were ready for one thing: war with the United States.

Japan’s eight key Mandates bases, each with air, fuel, and harbor facilities, shore fortifications and troops, gave her a head start when the war began. The bases increased both her defensive and offensive capabilities, screening her eastern flank on the one hand, and providing partial or full support for early attacks on Pearl Harbor, the Philippines, Guam, Wake, the Gilberts, and Melanesia on the other.

Though the pre-war bases helped Japan gain the initial advantage, they proved far from adequate for the long pull. Japan continued to fortify the Mandates and poured vast numbers of troops into the islands during the war, making the Mandates of 1944 far stronger than the Mandates of 1941. In mid-1944 the Marianas sector alone had nine operational airfields with a total capacity of 400 planes, compared to eight fields with about the same capacity in all four sectors combined when the war began. In 1944 the garrison of a single island, Saipan, came to 30,000, almost as many as the 36,000 in all Mandates garrisons in 1941.

What about the myth that Japan spent twenty years before the war building invulnerable island fortresses? Actually, she began the Mandates bases in earnest only two years before the war, and never finished them. Construction was still going full blast in 1944, when American forces smashed into the islands and seized or neutralized the bases. The Mandates were invulnerable no longer than it took the United States to dominate the sea and air around them.