Operation Causeway: The Planned Invasion of Formosa 1944 - Part One


As the Pacific War progressed, and as Allied forces moved closer to Japan, the US began to look at what targets should be considered for attack, in order to provide bases for the eventual invasion of the Japanese Home Islands. One that was considered early on was the island of Formosa (today, it's known as Taiwan). For a number of reasons, it was eventually dropped in favour of invading first, the Philippines and then Okinawa. Why was this, what were the plans for invasion and what might have happened had the US chose to invade?

War Plan Orange

War Plan Orange was one of a series of joint US Army and Navy plans for dealing with a possible war with Japan sometime between the First and Second World Wars. It was part of a broader series of color-coded war plans that outlined potential U.S. strategy for use in a variety of hypothetical war scenarios, such as Black (Germany), Red (British Empire) and Green (Mexico). The plans, developed by the Joint Planning Committee (which later became the US Joint Chiefs of Staff), were officially withdrawn in 1939 at the outbreak of World War II in favour of five Rainbow Plans developed to meet the threat of a two-ocean war against multiple enemies. The original War Plan Orange was developed by Rear Admiral Raymond P Rogers in 1911, but went through a series of revisions:

The plan assumed that the Japanese would capture the Philippines and other islands in the Pacific. The US would then build up its forces around Hawaii and the US west coast after which it would launch a counteroffensive across the Pacific towards the Japanese Home Islands, seeking a final decisive battle with the Imperial Japanese Navy (IJN) as per US Navy doctrine, based upon the ideas of Alfred Thayer Mahan. The counteroffensive would include amphibious assaults to recapture territory, including the Philippines, providing bases to support the next stage of the advance. Bases in the Philippines would then be used for the air and naval blockade of Japan or even to support an invasion.

The Pacific War

The actual outbreak of war and the early conduct of Japan's Pacific offensive to establish its Co-Prosperity Sphere (i.e. it's advances in the South Pacific and SE Asia) meant that War Plan Orange had to be modified, especially the timetable for the counteroffensive. Most importantly, the Japanese advances into New Guinea and the Solomon Islands, which threatened Australia, had to be checked. Therefore, the first major Allied counterattack in the Pacific War was at Guadalcanal (as well as the rest of the Solomon Islands) with Operation X, alongside General Douglas MacArthur attempting to clear the Japanese from New Guinea, with both campaigns proving victorious by the end of 1943.

This paved the way for the delayed Central Pacific advance to commence, with the attack on Tarawa Atoll (part of the Gilbert Islands) in November 1943 as part of Operation Galvanic. It was quickly followed by attacks on Kwajalein (January 1944, Operation Flintlock) and Eniwetok (February 1944, Operation Catchpole), with Saipan (June 1944) and Guam (July 1944) in the Marianas following later in the year as part of Operation Forager. All of these provided important air bases and anchorages for continuing the Pacific War plus Saipan provided bases from which B-29 bombers could start striking Japan.

This was where a serious debate over future American strategy really started, something that would happen on more than one occasion due to the lack of a single overall commander in the Pacific (unlike Europe). Admiral Chester Nimitz, Commander of the Pacific Ocean Area and responsible for the Central Pacific Theatre of Operations (TOO) wanted to bypass the Philippines altogether and invade Formosa. He argued that:

However, Nimitz remained open to a possible landing in the Philippines with the objective being to take Leyte and Mindanao in order to support the Formosa / Amoy operation, while bypassing Luzon and the capital Manila due to the anticipated scale of Japanese resistance. Nimitz was supported in his arguments by Admiral Earnest King (Chief of Naval Operations) and General Henry ‘Hap' Arnold, Commander of the US Army Air Force. General George Marshall (US Army Chief of Staff) on the other hand, recommended bypassing both Formosa and the Philippines and heading straight for the Japanese home islands. This viewpoint was probably influenced by the fact that operations in 1944 had demonstrated that US forces in the Pacific had grown to such a size, including their armada of support vessels, that the operate at significant distance from their main bases, seize enemy bases and turn them into staging areas for the next round of operations, while countering enemy land-based and sea-based airpower.

General Douglas MacArthur was the main advocate of an invasion of the Philippines, his reasons being both military and political:

What tipped the argument in MacArthur’s favour was that firstly, the Japanese launched their Ichi-Go (Number One) offensive, which pushed Chinese Nationalist forces away from the coast and captured many of the Fourteenth Air Force's bases, limiting the amount of home-based support that could be provided to an invasion of Formosa. Secondly, he was supported by the President’s Chief of Staff, Admiral William Leahy, principally for the political reasons given. That was sufficient to persuade President Franklin D Roosevelt and the other flag officers at the Pentagon, and so MacArthur was directed to invade the Philippines. That campaign began in October 1944 when US forces invaded Leyte, followed by Luzon in January 1945 and the liberation of Manila in March. The Philippines would be developed as a staging area for an invasion of Okinawa (Operation Iceberg) which in turn, would have become a staging area for the invasion of the Japanese home islands (Operation Downfall).

Operation Causeway

Interestingly, planning for Operation Causeway was relatively well advanced. A ‘Pacific Fleet and Pacific Ocean Area Joint Staff Study’ was being undertaken and updated as the broader Pacific campaign unfolded, right up until December 1944. It envisaged an operation covering both an invasion of Formosa and the capture of the port of Amoy on the Chinese mainland. It would have involved around 320,000 US Army and 100,000 US Marine Corps personnel with the US Tenth Army in overall command, controlling three corps: III Amphibious and XXIV for Formosa and V Amphibious for Amoy. The two separate American drives across the Pacific (Central and Southwest) would have had to unite (something which didn't happen until the invasion of Okinawa) with a target invasion date set no later than 15 February 1945. It would have been preceded by wide-ranging carrier-based and land-based air strikes against Japanese bases and airfields across the Western Pacific, with US naval forces (principally submarines) interdicting Japanese sea lines of communication to the island.

The first phase would start three days before the actual invasion (D-3) and involve a large-scale aerial assault against the island, followed by US naval units bombarding targets on the coast on D-2 and D-1. On D-Day, the two corps would have landed on beaches around the major port of Takao on the southwest part of the island. III Amphibious Corps, with two Marine divisions and one Army division, would land on the western flank while XXIV Corps, with three Army divisions would land on the eastern flank. Two divisions would be acting as a floating reserve. Once US forces had secured the initial beachhead, they would then advance north towards Takao. A secondary landing at Nan Wan Bay (in the south) would then be undertaken, where the wide, relatively flat beaches allowed for the unloading of ships without a port.

After Takao had been captured, US forces would continue north, capturing additional ports and airbases as they went, all of which would be rebuilt and put to good use. In truth, the plan did not require the capture of the entire island – US forces would advance to a line running roughly east-west (it's exact location and course were never determined) after which southern Formosa (with the help of US reinforcements that included seven engineer groups) would be turned into a staging area for future US operations, including an eventual attack on the Japanese home islands. The remainder of the Japanese garrison would be contained. As well as the sustainment of the attacking forces, any logistical considerations had to include some sort of provision for the civilian population. A major campaign such as this would inevitably lead to widespread collateral damage, include large numbers of civilian casualties and destruction of parts of the island's infrastructure, increasing the need for extensive medical and engineering resources. Finally, Phase II of Operation Causeway would start on D+20, with V Amphibious Corps striking at Quemoy Island and the port of Amoy. This would consist of three Marine divisions, bringing the total employed in Causeway to five out of the six Marine divisions then in the Pacific Theatre. There was also the possibility of using the Australian I Corps with at least two divisions. Comparing this with the invasion of Okinawa, the basic forces scheduled to be used in Causeway consisted of nine divisions, compared to the seven used in Iceberg. If the floating reserve and Australian divisions are included then that rises to fourteen divisions, compared to seven and with that a comparable increase in the logistical requirements for supporting such a force as well as rebuilding and repurposing the infrastructure in southern Formosa and around Amoy. These were to host somewhere in the region of eighty-five squadrons (of various aircraft types) and would project airpower in conjunction with naval airpower from eleven fleet, seven light and thirty-two escort carriers, as well as long-range bombers.

 The Japanese would, in all likelihood, have used their bases in the general area (Formosa, Luzon, China and probably the Ryukyus Islands) to attack the invasion fleet. An example of what could have happened is the Battle of the Philippine Sea on 19-20 June 1944, which started as a Japanese attempt to orchestrate the decisive naval battle they codenamed Operation A-Go. It was launched in response to the US invasion of Saipan as part of Operation Forager, located inside what the Japanese viewed as their 'inner defence perimeter'. Saipan was important as it could become a staging area for the next round of US operations as well as being able to base B-29 bombers, which from there could reach the Japanese home islands. A-Go pitted 450 Japanese aircraft flying from eight aircraft carriers and three hundred land-based aircraft against 900 US carrier-based aircraft. The battle was a significant defeat for the IJN which lost some 600 aircraft and three of its carriers compared to the US Navy losing around 120 aircraft. Even if the result had been significantly better for the Japanese, by this time American industry was producing ships and aircraft at such a rate that any losses would have been able to have been replaced relatively quickly. Following this, on 12 October 1944, the US Third Fleet attacked Japanese bases on Formosa to try and eliminate Japanese airpower there. This was conducted in support of MacArthur's invasion of the Philippines, which was starting on 20 October. Again, US carrier-based airpower triumphed, with the Japanese losing around 500 aircraft and the Americans around 100. What these two operations demonstrated was that a turning point had been reached – that there were now enough US carriers so that when concentrated in a large enough force, they could operate successfully against even massed Japanese land-based airpower. It meant that if the US chose to conduct an amphibious invasion of Formosa, it could do so outside of the range of US land-based airpower.

Japan's Strategy

In early-to-mid 1944, the Japanese Imperial General Headquarters (IGHQ) was re-evaluating its strategy. The Allied advances in the Central and Southwest Pacific campaigns had made enough progress so as to be on the verge of penetrating what the Japanese had deemed their 'final' defensive perimeter. In addition, Allied submarines were sinking Japanese merchant shipping and disrupting the floe of raw materials from the 'Southern Resource Area' (Indonesia and SE Asia). Finally, US air attacks had forces the IJN to move its main fleet away from the huge base at Truk, which more-or-less ceded control of the Central Pacific to the USA. The IGHQ therefore established a new defence zone, based on the triangle of Formosa, Philippines and Ryukyu Islands, laying plans to hold that zone at all costs. The IGHQ Staff then formulated four plans for victory (Sho-Go) in late July 1944 to fit in with that overall concept and to try and (once again) orchestrate a decisive naval battle.

Sho-Go I looked at a possible US invasion of the Philippines. Sho-Go II would be used to defend either Formosa or Okinawa (or possibly both), while Sho-Go III and IV covered the home islands. At the same time, the IJN was gathering together its remaining major fleet units into a single strike force, which, when combined with land-based airpower and sustained Japanese resistance on any and all occupied territory, would win the final 'decisive battle'. The IGHQ predicted (correctly as it happened) that the next major US target would be the Philippines and transferred two divisions there that were going to be deployed on Formosa. Still, the defence of Formosa remained a high priority as its loss would have serious repercussions for operations on the Chinese mainland and disrupt the LOCs with SE Asia. The IGHQ also started what would prove to be the final round of industrial and reserve mobilisation in the country. The population would be armed and trained for a last-ditch defence of the homeland, while the forces on the Asian mainland (Manchuria and China) were to become as self-sufficient as possible, by confiscating local resources. Finally, the IGHQ planned a change of tactics, one that was hoped, could bring about a decisive victory and change the course of the war.

The 'Divine Wind'

Up until early 1944, the Japanese believed that they could orchestrate a final 'decisive battle' with the US Navy and inflict sufficient losses on the USA that would force the Americans to seek a negotiated peace. After the defeat at the Battle of the Philippine Sea, it was obvious that the Japanese were unlikely to achieve that goal by conventional means. They therefore began to form 'special attack units' (special in this case meaning suicide), more commonly known as kamikazes, which included piloted aircraft, which were to be flown directly into enemy ships, the theory being they would have greater accuracy and greater kinetic energy causing more damage. The units also included manned boats, piloted torpedoes and a recently developed rocket-powered enhanced glider. There was some opposition to the plans within the IGHQ, which saw it as a waste of trained manpower and limited resources in a desperation move that would ultimately, have little impact on the final outcome. However, by this point in time, options were running out for Japan. The Empire was facing total defeat and needed to come up with a suitable response, however extreme. The main objective would be to run up Allied casualties to the point at which continuing the war would be judged as being too costly and they would be forced to come to the negotiating table. This meant a change in targeting priorities. Up to then, the main targets were the major US Navy fleet units such as aircraft carriers and battleships – large scale casualties in these would help Japan re-establish parity in naval combat power and derail the US plans.

The events surrounding the battles of the Philippine Sea, Leyte Gulf and the air campaign against Formosa showed that the US held an almost unassailable position both tactically and operationally. All that remained was to try and influence the war at a strategic / political level by inflicting high enough casualties to affect the US home front. The new tactics would focus Japan's remaining air power (both conventional and kamikaze) on attacking the transport fleet and amphibious ships. Casualties here would reduce the number of troops as well as the amount of equipment and supplies that could be landed in both the initial assault and in follow-up operations. In addition, transports were more vulnerable to attack than the heavily armed and armoured US Navy fleet units. The Imperial Japanese Army (IJA) were also developing new tactics to fight invasions. Originally, the idea was to set up a forward defence to meet and defeat US forces as they landed before they could build up the combat power to advance inland. In theory the plan was sound but in practice it couldn't work in the face of overwhelming US naval gunfire and air support. Now, the IJA planned to conduct a delaying action on the beaches while the main lines of defence would be inland, where Japanese forces could build extensive networked fortifications, taking advantage of both terrain and natural obstacles. US ground forces would be forced to wage battles of attrition which would increase casualties and wreck their timetable. Indeed, on the larger islands and archipelagos (such as Formosa and the Philippines), Japanese positions in the interior would be out of range of US naval gunfire support. Japanese forces were also forbidden by the IGHQ to conduct banzai charges, mass infantry attacks designed overwhelm an enemy defensive line at a critical point by sheer weight of numbers. Effective in China, they almost always ended in disaster during the Pacific War and were being used more and more not to achieve victory but to escape the shame of defeat. Japanese troops were instead to hold prepared positions (such as natural caves turned into bunkers and connected via tunnel networks) while conserving ammunition and supplies, counterattacking at the most opportune moments.

The new tactics were tested during the Battle for Peleliu (September – November 1944). US forces eventually captured the island, but it was one of the bloodiest battles of the Pacific War. The US liberation of the Philippines started on 20 October 1944 and saw the first large scale deployment of Kamikazes, the first attack of its kind being on 25 October. It caught the US Navy off-guard, sinking and damaging several surface vessels. The US navy developed tactics to counter the Kamikazes including around-the-clock combat air patrols, increasing the number of anti-aircraft guns aboard ship, and posting destroyers on the edge of task forces to act as radar pickets. These tactics helped to stop the Kamikazes from becoming a decisive factor, but the attacks continued throughout the rest of the campaign. The lack of accurate feedback as to how effective they actual were meant that their HQ would generally exaggerate the amount of damage inflicted in the attacks. For example, while numerous aircraft carriers were reported sunk, in fact only three escort carriers were sunk, with twenty-one other carriers and twenty-four battleships being damaged. While not insignificant, time and again many of the ships were quickly repaired and put back into service.


The island lies with 100 miles of the Chinese mainland and is around 350 miles in length (north to south) and ninety-three miles across (East to West) at its widest. The west coast is probably the easiest to land on but still provides some obstacles in the way of sandbars, mudflats and rice paddies. The east coast has a range of mountains the run north to south that would have been an obstacle to movement and where Japanese forces could build numerous fortified positions. At the time, the population of the island totalled some 5.8m which included Japanese, Chinese and native inhabitants. Having occupied the island for some fifty years, the Japanese had built a significant infrastructure on the island including a substantial road and rail network and two good ports (Takao and Kirun), both of which had been fortified. Formosa had also been industrialised, much of which could have been turned to providing direct support for the defending forces and was self-sufficient in food and water and could therefore support a significant garrison almost indefinitely, unlike many other Japanese island strongholds in the Pacific. US intelligence estimates put the number of Japanese troops at around 170,000 (100,000 were IJA with 70,000 being IJN), many being in service and support units but who could have been formed into ad hoc combat formations if necessary. The Japanese civilian population would have supported the defence of the island or potentially conducted guerrilla warfare behind US lines in the event of an attack. In a similar vein, many Formosan native inhabitants volunteered to serve with Japanese forces in the Pacific War and so could have been counted to have supported the defence of the island. The Japanese forces on Formosa were under the command of the 40th Army, headquartered in Taihoku (Teipei), itself part of the Tenth Area Army (a Japanese army was equivalent to a Western corps, while an area army is equivalent to a Western army). These forces included five infantry divisions (9th, 12th, 50th, 66th, 71st), six independent mixed brigades (which varied in strength from a regiment to a small division) and numerous battalions and companies armed with various automatic cannons, artillery pieces, machine guns and anti-aircraft guns. There were also logistic, engineer and labour units that could be converted into ad hoc combat units, as well as naval guard units defending the ports. All this would have added up to a formidable challenge for Allied forces but given their overall military superiority, the island would have inevitably been taken, the only questions being how long would it take and how many casualties would the Allies have suffered?

Historically, the US secured Luzon towards the end of February 1945, with the capture of the fortress of Corregidor. While some Japanese forces remained active, they were effectively quarantined by US ground troops. The IGHQ assumed that the next Allied attack would either be against Formosa or Okinawa, with the home islands to follow whatever the choice. In actuality, the US invaded Okinawa with Operation Iceberg, supported by the Royal Navy's Pacific Fleet. But what might have happened had the US decided to attack Formosa instead? In all probability, the course of the campaign would have mirrored that on Okinawa. Allied air and naval power would have isolated the island, softened up the defences with ground troops landing and slogged their way inland. The Japanese would have launched both conventional and kamikaze attacks against the invasion fleet, which would have been hindered (but never completely negated) by anti-aircraft cover and combat air patrols. The IJN may also have launched their last great special naval operation of the war, Ten Go, with a naval task group centred around the battleship Yamato making a sortie towards the invasion beaches. In reality, Allied airpower and submarines sank the Yamato and its escorts before it reached Okinawa. Given the distance from Japan to Formosa is several times that of Japan to Okinawa, it is unlikely the Yamato would have managed to get anywhere near Allied forces. Once they had established a beachhead, US ground forces would have advanced inland either to a stop line halfway up the island or the decision may have been taken (depending on the level of casualties suffered at that point) to go on and clear the island of Japanese forces to prevent Japanese counterattacks and continued kamikaze strikes from hidden airfields. It would have also been very attractive to seize additional ports in the north and turn them into bases for the US Navy. Once the island was secure, it would have been used a staging base for the next round of operations – the attack on the Japanese home islands, codenamed Operation Downfall, the first phase of which was tentatively set for November 1945.

Causeway Logistics

Interestingly, US planners had provided preliminary estimates for the logistic requirements of Operation Causeway before it was cancelled. Most of the supplies needed would have had to be transported by sea, although there was provision for both airlifting supplies and local procurement of resources. Such a supply chain would have extended all the way back to Hawaii via bases in the Marianas, Carolines and Marshalls. If one of the islands in the Philippines had been secured, then that would have been used as well. Pre-war planning (and wargaming) had suggested that a fleet could only operate some three weeks away from its bases. However during the war, the US Navy found that timeframe had to be extended. The answer lay in developing and expanding at-sea logistical support through what became known as the 'fleet supply train'. These were ships that could provide for the refuelling and repair of other ships, hold ammunition and cater for the evacuation of wounded personnel. Support ships such as these were usually organised into task forces known as 'Service Squadrons' or 'Servrons'. They also needed to carry materials, equipment and supplies to support the initial operations ashore so when an island had been secured, a (forward) base could be constructed by moving prefabricated docks into position and then utilising Army, Marine and Naval construction battalions (see the John Wayne film 'The Fighting Seabees') equipped with equipment such as bulldozers, graders and handheld power tools.

Once built, supplies could be landed, organised and moved inland. In the case of Formosa, it was planned that Takao would become the main port. It had a docking capacity of ten ships, and could accommodate another three ships within the harbour, as well as having a good infrastructure including mechanical cranes. The nearby smaller ports of Toshien and Anping could have provided additional docking capacity but were by themselves, not sufficient to support Operation Causeway. It was expected that the Japanese would sabotage as much of the port's operation as they could, so the US would have had to undertake a substantial engineering and repair effort to get them back into operation as quickly as possible. While this was happening, the US would have had to land troops, supplies and equipment directly onto the beaches. Nan Wan Bay, on the island's southern coastline, could accommodate up to thirty ships using this method. However, US forces using this method had to be careful as overuse could physically wear out the beach, plus cranes were still needed to unload heavy equipment. All this underlined the importance of seizing Amoy, just across the Formosa Strait from Formosa itself. The port would have provided substantial docking capacity within easy support range of the island. Supply estimates on the amount of tonnage to be unloaded were as follows:


Month 1

Month 2

Month 3





Amoy Area




The initial assault force to land on Formosa had a strength of around 163,000 personnel, with another 141,500 arriving as reinforcements. For Amoy, the initial assault force amounted to 73,000 with another 46,000 arriving as reinforcements. The monthly supply needs of the forces ashore were estimated to be 253,000 tons for Formosa and another 99,500 tons for Amoy. The supply requirements for the initial assault were 733,000 tons for Formosa and 330,000 tons for Amoy. All this would require the construction of a vast network of bases, depots, magazines, supply dumps, refuelling points, maintenance facilities and airfields, along with utilities and a water supply, as well as repairing the road / rail networks and organising supply convoys. In addition, was question as to casualties. The first two months of combat were expected to result in some 7,500 killed in action, 7,500 wounded badly enough to require local hospitalisation and around 22,000 wounded who would require evacuation. Roughly two thirds of these would be from the fighting on Formosa, the rest from the fighting around Amoy.


If the US had invaded Formosa rather than Okinawa, there would have been both sort and long-term consequences. Firstly, the establishment of US bases would have created a major forward US presence in the region, equivalent to what both the Philippines and Okinawa became historically. Secondly, such a presence would have been vital in projecting US military power and having bases both there and on the Chinese mainland around Amoy would have made it easier for the US to directly aid the Chinese Nationalists under Chiang Kai-Shek during the Chinese Civil War, which resumed after the Japanese surrender. Thirdly, having China as the principle focus of US strategy in 1944-45 could have mitigated against the Japanese Ichi-Go offensive coming to a successful conclusion and left the Nationalists in a much stronger position when the Civil War resumed. Fifth, such as emphasis on China might have meant there was no invasion of the Philippines, or at least a significantly reduced effort, with possible negative consequences in the post-war period. MacArthur not only promoted the liberation of the Philippines for military reasons (to use a staging area for the invasion of the Japanese home islands) but political ones too. By fulfilling his promise to return, he could more-or-less guarantee the Philippines support for the USA in the coming Cold War. Even so, a Nationalist victory in the Chinese Civil War would have radically altered the balance of power in favour of the West at the start of the Cold War. It is unlikely there would have been a Korean War and with no Communist bases and sanctuaries, the Viet Minh would have found it very difficult to beat the French in Indochina. As a result, and longer term, there would have been no need for direct US intervention in Vietnam. On the other hand, if it had become directly involved in the Chinese Civil War while supporting the Chinese Nationalists, it could have become entangled in a major foreign war across mainland China.

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How to cite this article: Antill, P (11 April 2023), Operation Causeway: The Planned Invasion of Formosa 1944 - Part One , http://www.historyofwar.org/articles/operation_causeway_formosa_1.html

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