These are a pair of essays written by Brad Smith III in August 2006. I felt that they deserved a wider audience and have converted them into a History and Technology entry.

The Limits of Japanese Strength in December, 1941

In December of 1941, the Japanese Imperial Army (IJA) and Navy (IJN) mounted a series of combined operations that reached from Wake Island in the east to Bangkok, the Isthmus of Kra, and northeastern Malaya in the west. These operations were an impressive display of Japanese planning and operational capabilities, and the very judicious allocation of limited combat power.

However, the sheer breadth of the offensives - which were the maximum amphibious effort the Japanese military managed during the entire war - obscured some very real weaknesses in the Japanese forces, their doctrine, and their leadership.

First and foremost, the Japanese operations were all against opponents that were significantly weaker in 1941, both in terms of forces in the theater and available for redeployment from elsewhere, then at almost any other time during the conflict. The British were fully occupied in the defense of the British Isles and in the Atlantic and Mediterranean; India, Australia, and New Zealand were all still essentially mobilizing, and their combat-ready forces were deployed in Africa and Middle East. The Dutch, with the Netherlands occupied, were even more limited. The United States, which had begun mobilizing in the fall of 1940, was simultaneously equipping the large American forces being mobilized, preparing them for contingency operations in the Atlantic and ETO, and committed to supporting the European Allies - US Army and Navy forces in the Pacific, although significantly stronger then they had been in 1940, still came farther down the priority list than those forces at home or in the Atlantic.

Secondly, it is worth understanding that the Japanese rarely, if ever, chose to mount an opposed amphibious operation, unless the nature of the target demanded it. Generally, the IJA forces were landed at undefended locations, and then marched overland to their targets. The only significant exception to this among the December operations were the 1st and 2nd Wake assaults, and 1st Wake is notable for being among the very few World War II amphibious operations that was decisively defeated at the water's edge.

Third, a review of all of the December operations reveals just how scant Japan's resources, in both landing forces and the shipping necessary to move and sustain them, were in 1941. The demands of an industrialized island nation for shipping are always large, and the Japanese merchant marine was incapable of meeting them, even in peacetime. What with the ongoing war in China, the need to maintain large and expensive ground and air forces in Manchuria and Korea to guard against the Soviets, and the occupation of French Indochina, an already thin asset had to be spread even farther, thus limiting what shipping was available for the Pacific offensives.

As it was, when the December operations were planned and then mounted, the available amphibious forces - meaning both troops and shipping - were relatively small. The total ground force allotted by the Army and Navy to the Pacific offensive amounted to four corps-level headquarters and about 12 division equivalents, something less than a quarter of the IJA's total ground forces, and a corresponding number of IJAAF air units. In these operations the IJNAF provided the bulk of the air power, including the land-based aircraft, used by the Japanese.

Of those 12 division equivalents, moreover, it appears that no more than about a third - perhaps four division equivalents, or about 12 brigade/regimental combat team equivalents - were ever afloat simultaneously in December, 1941.

The specifics of the Japanese offensives were as follows:

  1. SHANGHAI Operation: IJA and IJN forces cooperated to occupy the International Settlement in Shanghai, China; the forces allotted amounted to small elements of the existing Japanese forces in China and IJN forces already assigned to the China Station.
  2. HONG KONG Operation: The IJA's 38th Division, strongly reinforced by other elements of the Japanese forces in China, moved overland to attack the British colony. The troops were supported by a relatively small IJN task force.
  3. THAILAND Operation: The IJA's 15th Army (a corps equivalent) invaded the independent but neutral kingdom overland from occupied French Indochina, with the Guards Division as the spearhead. The only amphibious element was a battalion combat team built around the 3rd Battalion of the 4th Guards Regiment, which departed Saigon on 5 December 1941 aboard one transport and landed at Bangkok three days later.
  4. MALAYA Operation: The IJA's 25th Army (corps equivalent) invaded southern Thailand and northeastern Malaya (then a British colony) with four brigade- to division-sized forces, included a substantial force of IJAAF ground units to operate the captured airfields; these forces included:

    • KRA force: a brigade-sized force built around the 143rd RCT, detached from the 55th Division, which landed on eastern side of the Isthmus of Kra. This force left Saigon on 5 December 1941 aboard 7 transports and landed just before midnight on December 7th.
    • SINGORA force: the reinforced 5th Division (built around the 11th and 41st RCTs, plus divisional- and corps-level artillery, armor, engineers, etc.), aboard 12 transports, which left Hainan on 4 December 1941 and also landed just before midnight on December 7th.
    • PATTANI force: a brigade-sized force built around the 42nd RCT, detached from the 5th Division, aboard six transports, which also landed on December 7th.
    • KHOTA BHARU force: a brigade-sized force built around the 56th RCT (detached from the 18th Division), aboard three transports, which also landed on December 7th.

  5. MALAYA SECOND WAVE: additional army elements and supply and support ships, some of them reloaded vessels of the initial convoys, left Cam Ranh Bay on 13 December 1941 and arrived off their respective beachheads on December 16th and 17th.

  6. BORNEO Operation: A brigade-sized force built around the IJA's 124th RCT (detached from the 16th Division) and the battalion-sized 2nd SNLF, aboard 10 transports and support vessels, left Cam Ranh Bay on 13 December 1941. The force landed troops in Borneo on December 16th and occupied Miri, Seria, and Lutong; six days later, elements of the same force were loaded aboard the same transports and landed at Kuching (Sarawak) on December 23rd.
  7. PHILIPPINE OPERATION: The IJA 14th and 16th armies (both corps-equivalents) each provided a reinforced division and corps troops for the Philippines invasion, which proceeded in two stages against targets on Luzon and Mindanao (the two largest and northernmost and southernmost islands, respectively, of the archipelago): The first phase included:
    • LUZON STRAIT force: Small (battalion-sized or less) elements of the IJN aboard a two large and several smaller transports and support vessels left Taiwan and occupied the Luzon Strait islands (Batan and Camiguin) on December 8th and December 10th, respectively.
    • APARRI force: An understrength regimental-sized force (about 2,000 men drawn from the 2nd Taiwan (Formosa) RCT, part of the 48th Division) aboard seven transports, left the Pescadores (south of Taiwan) on December 7th and landed at Aparri and Gonzaga (far northern Luzon) on December 10th.
    • VIGAN force: An understrength regimental-sized force (about 2,000 men also drawn from the 2nd Taiwan RCT, part of the 48th Division) aboard five transports, left the Pescadores (south of Taiwan) on December 7th and landed at Vigan (northwestern Luzon) on December 10th.
    • LEGASPI force: A regimental-sized force (3,000 men drawn from the 33rd RCT, part of the 16th Division, and the 1st SNLF) aboard six transports, left Palau (Western Carolines) on December 7th and landed at Legaspi (far southeastern Luzon) on December 12th.
    • DAVAO force: A brigade-sized force (5,000 men, built around the 146th RCT, part of the 56th Division) aboard 14 transports left Palau on December 17th and landed at Davao (southern Mindanao) on December 20th; a day later, a detachment (about two battalions) was re-embarked and sailed for Jolo, in the Sulu Islands (far southwestern Philippines) landing there on December 24th.
    • LINGAYEN force: A large force (43,100 men) that included the bulk of the 14th Army (34,900 men), logistics personnel (4,600 men) and IJNAF personnel (3,600 men), the ground combat element included the 48th Division, built around the division's own 1st Taiwan and 48th RCTs, and the attached 9th RCT (detached from the 16th Division) plus divisional and corps-level artillery, cavalry, armor, and engineers. Loaded aboard some 60 transports, freighters, and support vessels, with about 150 purpose-built and 50 extemporized (described as "motorized sampans") landing craft embarked, the force left ports in the Pescadores and Taiwan on December 17th and 18th and began landing troops at four separate points on Lingayen Gulf (Central Western Luzon) on December 22nd through the 24th.
    • LAMON BAY force: A reinforced brigade-sized force (7,000 men drawn from the 16th Division, including the 20th RCT and division troops) left the Ryukyus on December 17th aboard 24 transports and began landing at Lamon Bay (Central Eastern Luzon) on December 24th.

  8. GUAM Operation: A reinforced brigade-sized force, the Army's 144th RCT, (roughly 5,000 men) aboard nine transports, landed on Guam on December 8th and rapidly overran the island.
  9. WAKE Operation: An SNLF battalion, aboard four transports, left Kwajalein on December 6th and attempted to land on Wake on the 10th; it was repulsed (First Wake). The survivors, now reinforced to a strength of 2,000 men with additional personnel and shipping (six transports total) drawn from the IJA Guam occupation force, landed on December 22th and forced the island's surrender the next day.
  10. GILBERTS Operation: Very small (company-sized or smaller) IJN elements left Jaluit on December 8th aboard small naval vessels and landed on Tarawa and Makin Islands on December 10th.

So the grand total was approximately 14 brigade/regimental combat team equivalents, or - very roughly - four triangular infantry division equivalents and supporting elements, all only partly motorized. The Japanese infantry was, for the most part, true leg infantry, and horse-drawn artillery and logistics elements were still very much part of the IJA.

This is pretty impressive for 1941 (the Allies would not manage multi-division landings until TORCH in 1942), but it is also the largest amphibious effort the Japanese managed, historically, throughout the war. The largest subsequent amphibious operations in the Netherlands East Indies (NEI) (the invasion of Java, for example) were no larger than corps-divisional level, and often smaller, at the brigade, regimental, or battalion scale. The reality is that Japan's greatest amphibious effort during the war amounted to the roughly simultaneous movement, landing, and support of four reinforced divisions - and this at the ranges and sea states common in the Western and Southwestern Pacific in December.

The ultimate point, of course, is that given the sheer size of the Pacific, the strength (however limited) of the Allies in their major bastions, and the relatively small forces available for the December offensives, anyone attempting to detail an alternative yet realistic strategy for the Japanese in the winter of 1941-42 has to consider the historically finite resources that were available to Japan. Otherwise, such speculation is simply fantasy.


"The Rising Sun in the Pacific" by SW Morison; Vol. III of "United States Naval Operations in World War II."
"The Fall of the Philippines" by Louis Morton, one of the US Army Official History volumes (the "Green Books")
"Chronology of the War at Sea" by Jurgen Rohwer and Gerhard Hummelchen
Leo Niehorster's World War II Order of Battle webpage

Japanese Strategic Alternatives in December, 1941

The first question that has to be asked in examining whether alternative Japanese operations during the first month of the Pacific War would have led to a more advantageous strategic situation (for the Japanese) is: "What is the ultimate objective?" Possible answers are:

  • A stronger defense perimeter for the inevitable American counteroffensive?
  • A faster conquest of Malaya and the NEI, which was, after all, the strategic point of going to war?

Obviously, if the objective is a faster conquest of the British and Dutch colonies in the Western Pacific, all of the Japanese operations east of the Dateline, including the Oahu raid, were ultimately counterproductive.

If, however, the objective was to create a stronger perimeter, then it is worth examining what each of the American outposts in the Pacific provided in a strategic sense, both for the US war effort and against the Japanese, and what intelligence - if any - the Japanese were likely to have about the garrisons of the US outposts. Worth pointing out is that four of the six outposts were military reservations, without any permanent civilian population, so the Japanese would have had no opportunity to secure human intelligence. Of the two islands with civilian populations (Guam and Samoa) only Guam appears to have had a Japanese population, who were promptly interned the day the war began. It is fair to infer that Japanese intelligence on all six outposts was likely to be limited. The other point to make is that simultaneous operations are more likely to succeed in taking over these outposts swiftly than are operations taken in series, which opens the possibility of US reinforcements reaching the islands before the Japanese are successful.

In (rough) terms of distance from Japanese possessions, the US Pacific outposts were:

  1. Guam
  2. Wake
  3. Johnston
  4. Midway
  5. Palmyra
  6. American (Eastern) Samoa

All were garrisoned, although their respective strengths and potential varied significantly; in addition, the US transpacific cable ran from Oahu to Midway to Wake to Guam to Manila. It is well worth remembering that in an era without instantaneous satellite communications, oceanic telephone cables provided a swift and secure method of communication and so were strategic assets. In addition, while all six islands had some capacity to support aircraft, none were really fully operational airbuses; Midway and Wake came the closest, but even their capabilities were limited when the Japanese attacked.

GUAM: A few minutes flying time from the Japanese island of Saipan, Guam was garrisoned by fewer than 500 US Marines and Chamorro police and militia; the island had no coast defense weapons, no air defense weapons or aircraft, and next to no assigned naval forces (one minesweeper and a station tanker). Guam did, however, have a large naval radio station, which by definition provided a radio direction finding and signals intelligence asset to the US. The naval station personnel numbered roughly 300, meaning the entire garrison was about 800, of which perhaps half were adequately armed even with individual weapons (rifles and pistols); the only crew-served weapons were the Marines' .30 machine guns. Interestingly enough, even though the defenses were very weak, the Japanese still chose to invade the island with a reinforced brigade (almost 6,000 men) strongly supported by land-based air and naval forces, with a naval covering force that included four heavy cruisers.

WAKE: Wake had a Marine defense detachment, an understrength Marine fighter squadron, and no naval support, but did have a radio installation and a large civilian construction detachment; the first Japanese attack included four transports (two AP/AK and two APD) carrying a reinforced IJN landing force battalion and fairly strong covering force (including three light cruisers), plus land-based air support from the Marshalls, and yet the IJN attack was repulsed, with the IJN losing two destroyers to Marine aircraft and coast artillery, plus damage to other vessels. The second Japanese assault included a reinforced RCT and greatly increased naval support, including two fleet carriers and four heavy cruisers.

JOHNSTON: Johnston had a Marine defense detachment, a construction detachment, a flight of PBYs (2 aircraft), and (on 7 December 1941) a naval force of six warships, including a heavy cruiser and five destroyer types. The airfield was still under construction at this time, and was estimated at roughly two-thirds complete.

MIDWAY: Midway had a full Marine defense battalion, a construction detachment, a squadron of PBYs (seven aircraft), and a functioning airfield, plus could be reinforced quickly via air from Oahu-Kauai.

PALMYRA: Palmyra had an understrength Marine defense detachment, and a construction detachment; the airfield was under construction and estimated at roughly two-thirds complete.

SAMOA: American (Eastern) Samoa had a full Marine defense battalion, a Marine reserve battalion, a construction detachment, and a functioning airfield; it also had a Navy observation squadron detachment.

The problem for the Japanese, of course, is with limited intelligence, they can't really know what the US garrisons on each of the six outposts is going to be when the IJN drops anchor. If the goal is to swiftly integrate these outposts into a defense perimeter, the Japanese cannot face a lengthy mopping up campaign, so they should go in over strength. To be safe, they have to assume a worst case, so estimate at least a US Marine defense battalion and composite squadron equivalent on Guam, Wake, Johnston, and Palmyra, with a Marine brigade and air group (2 squadrons) equivalent on Midway and Samoa. That would suggest - based on the standard 3-1 requirement for an assault - allocating a regimental combat team to the first four and a square brigade or triangular division to the last two.

Now, based on what historically the Japanese used at Guam and Wake (1st and 2nd attacks) I would expect the Japanese would need at least a regimental combat team for Guam and the equivalent for Wake (six APs/AK and 2 APDs each), with strong naval support - including carrier-based air - to successfully storm Wake Atoll; likewise, I'd expect they would need at least a square brigade (meaning two RCT equivalents, so a dozen APs/AKs at least) for Midway and the same for Samoa, again with strong naval support that included at least two fleet carriers. Johnston and Palmyra would require at least a battalion combat team, although there would be a real danger of a 1st Wake rerun.

Bottom line, if I were the Japanese and wanted to guarantee swift and nearly simultaneous seizures of the six US outposts, I'd be allocating at least six infantry Regimental Combat Teams (RCTs) and two separate Battalion Combat Teams (BCTs), or a total of 20 BCT-equivalents; basically, two reinforced divisions, which would be roughly analogous to the forces the Japanese used to invade Luzon, and so would require roughly the same commitment of shipping.

As far as Fiji goes (Suva is a city, not an island), the New Zealanders had a full infantry brigade (the 8th) there, along with some separate units and a composite squadron. The Japanese would need at least a division to take the archipelago.

Guam was a necessary objective for the Japanese, given its very real significance as a communications and potential signals intelligence center; the same for Wake. The Gilberts were essentially undefended, which is why the Japanese were able to occupy them with ships' company detachments. That being said, they did provide a southeastern outpost for the Japanese bases in the Marshalls and a potential eastern outpost for Nauru and Ocean, which did have some economic resources for the Japanese.

Basically, unless the Japanese were willing to delay any operations against Luzon until the spring of 1942, I don't think they had the shipping and assault troops to attempt anything beyond what they did historically, and even then they were gambling; the 1st Wake assault force was certainly understrength, as the Japanese learned to their cost.

As an aside, my understanding is the Japanese (56th RCT) succeeded at Kota Bharu against the 8th Indian Brigade primarily because of the strong air support provided to the Japanese and the fact the 8th Indian Brigade (which was made up of four Indian infantry battalions and a single battery of field, not coast, artillery) was dispersed along 30 miles of beaches and covering three airfields; as it was, the IJA troops were able to concentrate against a single IA infantry battalion (17th Dogras, IIRC).


"The Rising Sun in the Pacific" by SW Morison; Vol. III of "United States Naval Operations in World War II."
"Pearl Harbor to Guadalcanal" by Hough, Ludwig, and Shaw; Vol. I of "US Marine Corps Operations in World War II."
"The Fall of the Philippines" by Louis Morton, one of the US Army Official History volumes (the "Green Books")
"Chronology of the War at Sea" by Jurgen Rohwer and Gerhard Hummelchen
"Seventy Days to Singapore" by Stanley Falk
Leo Niehorster's World War II Order of Battle webpage
The NZ World War I Official History, available at the "NZTEC" website

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