* World War II Japan American invasion planning








World War II: American Invasion Planning--Operation Downfall (1945-47)


Figure 1.--The template for planning the invasion of Japan wa Okinawa. The montaneouds terraine was similar. And the Japanese Army was expected to fight to the death as they did on Okinawa. And the Americans noe knew that the Japanese Army would kill their own cuivillans if the trujed to surrender. On Okinawa civilians were horrified that the Army would kill them aor fiorce them to commit suiside. Only if they reached American lines woud they survide. Now the Japanese woukld go a step further, forcing civilians (including the women and children) to fight mericans tanks with shrpened bamboo poles if necessary--Ketsugo.

The last step in the war against Japan was the invasion of the Japanese Home Islands. Based upon Japanese resistance on one after another Pacific islands, American planners believed that Japan would never surrender and a climatic, bloody invasion would be needed--Operation Downfall. This was the proposed Allied invasion plan. There were two elements: Operation Olympic and Operation Coronet. Operation Olympic was scheduled for November 1945. The goal was to capture areas of the southern-most main Japanese island--Kyūshū. Invading the northern island of Hokido was considered, but discarded, largely because of the inclemnet weather. Kyūshū was the obvious objective because newly won bases in Okinawa could provide staging areas and air cover. The Japanese easily predicted the target. All they had to do was to calculate the range of air cover from Okinawa. As a result, the Japanese Army began to heavily reinforce Kyūshū. Next would be Operation Coronet in early-1946. This would involve the invasion of the Kantō Plain, near Tokyo, on the main Japanese island of Honshu. Airbases on Kyūshū captured in Operation Olympic would permit land-based air support for Operation Coronet. Air support was central in American military operations. Gen. Marshal advocated an invasion and Adm. Nimitz supported him. Gen. MacArthur was assigbned to command the invasion and began the planning. His plan provided that once Kyushu was secured, that air bases there could cover more northerly landings close to Tokyo. Had Downfall been launched, it would have been the largest amphibious operation in history, dwarfing the D-Day landings which had the advantage of being just a few miles across the Channel. In addition to the huge logistical issues, Japanese geography and terraine made these invasions a daunting prospect. Japan is a very mountaneouss country. There were few flat plains making sweeping armored thursts possible. A fight for the Home Islands woukd be more like Okinawa, meaning huge American casualties and much larger Japanese losses, in both military personnel and civilians, not even including the fact that Japanese civilans were already approachiung starvation. The Japanese were palnaning a massive defense of Kyūshū. American casualty estimates ran up into the millions. The Japanese casualties which had been 10 to 1 in just military personnel on Okinawa would have been astronomical on the Home Island. And the Japanese as part of Operation Ketsugo planed to use civilians to resist the invasion. The American military fully expected, and had every reaon to do so, that the Japanese Army would resist to the death just had they had done on Okinawa. In addition, the Japanese had amassed large numbers of aircraft which could be used in Kamakazi attacks on any invasion fleet. Many Japanese today maintain that Japan was near defeat and ready to surrender. It is certainly true that the military situation for Japan was hopeless and the population approaching starvation. This does not mean, however, that the Imperial Army was preparing to surrender. The situation had been hopless in every island fight after vGuadakcanl and the Japanese still fought bto the death. Ultra intercepts reported that the Japanese were strenthening their defenses in southern Kyushu. The Army High Command deployed three divisions there (June 1945) and that had been increased to 9 divisions (July 1945). Reinforcenents were brought back from China and Manchuria. In addition, the Army had incouraged, even forced, civilians to commit suiside as on Siapan and Okinawa. Some observers are convinced that Japanese civilian casualties in any invasion of the Home Islands would have been the greatest civilian disaster of World War II. Millions of Japanese soldiers and civilians may have died if America had invaded Japan. Ultra intercepts revealed constant strengthening of forces on Kyushu which reached 13 divisions. Both Marshall and Nimitz began to have second thoughts. Planners began to consider alternatives like poison gas, atomic bombs, or changing the invasion site. MacArthur continued to insist on Kyushu. [Giangreco, Hell]

Soviet Involvement

Given the intendity of Japanese resistance in one Pacific Island after another. American planners considered how to limit casualties. One alternative was to simply blackade and bomb Japan and slow down further offensive operations until Soviet entry into the War. Soviet participation has been a major American policy goal. And this did niot begin to channge until the Trinity Test of the atomic bomb (July 16, 1945). Stalin had committed to enter the war against Japan at the Tehran Conference (Novemnber 1943), but only after the Germans had been defeated. U.S. Army Chief of Staff, General George C. Marshal, rejected this idea (October 3, 1944). He feared that it would harm relatioins with the Soviets which had done the bulk of the land combat with the Germans. Marshall was concerned that Stalin would conclude that 'we are manuerving to get them into the fight in such a manner that they would suffer major losses.' Stalin and Molotov had both expressed concerns that the Western Allies were fighting the War with "Russian blood and American equipment". This of course was rather disingenuous given that for nearly 2 years the Soviets had been a NAZI ally and the Western war effort had forced Germany to divert much of its industrial production away from the Ostheer. But hoilding the anti-NAZI coaltion together was a real concern at the time. Marshal's war winning concept was a one, two punch. A massive Soviet invasion of Manchuria followed by American landings on the Home Islands. Soviet participztion was discussed at a neeting between Stalin and Ambassafdor Harriman (October 16). The Chieff of the U.S. Military Mission, Gen. John R. Dean attended. Stalin renewed his commitment and even offered a timetable 2 1/2 - 3 months after the Germans were defeated. This of course was all secret, but it would be the basis for American military planning. The primnary Soviet concern was that the Pacific Route was the primary conduit for American Lend Lease supplies. And this meant that meaning the vulnerable Trans-Pacic Railway was vital. The Soviets were concerned that the Japanes might strike the rail link if they knew that the Soviets were preparing an invasion. As a result, it would be up to the Americans to supply much of the fuel, food, trucks, and other supplies a Soviet invation force required. Stalin lost no time in presenting the Americanms an exhaustive list of what he wanted. It was massive, but the United States moved to fulfill it under a secret expansion of Lend Lease--Milepost. The Soviets expressed a concern over the security of the Pacufic route. The Americans unwilling to divert natiinal escoryts offered to ass=ist them with some shipos and amphibious landing ctaft. A base was established at Cold Bay Alaska for this purpose. Here 12,000 Soviet naval opersionnel were trained as part of Operation Hula to opetate the 149 American mine sweeprets, sub chasers, frigates LCI ammfiuvious landing cradt, and floating workshops. [Giangreco, Talk]

Final Step

The last step in the war against Japan was the invasion of the Japanese Home Islands. Based upon Japanese resistance on one after another Pacific islands, American planners believed that Japan would never surrender and a climatic, bloody invasion would be needed. This was the proposed American invasion plan. The earliest concept was for an invasion of the Home Islands in 1947 or 48. American success had, however, pushed up the time table to fall 1945. The first polans were submitted (June 1944). This was Operation Downfall which by August 1945 was in an adanmced stage of prepartion.

Two Componentss

There were five principal Japanese Home Islands. American planners for the invasion focused on three. Hokkido was eventually duscarded in favor of a southern campaign. Amnerucan planners focused on Kyūshū and Honshu. There were thus two components to Downfall: Operation Olympic (Kyūshū) and Operation Coronet (Honshu). . Invading the northern island of Hokido was considered, but discarded, largely because of the inclemnet weather. Hokkaido was atttactive becuse of the extent of the buildup on Kyūshū as well as the logistical train alreasy established throgh the Central Pacific. But given the importance of air power in American battle planning, the snow and inclemet weather argued against a northern invasion. The initial goal was thus to capture areas of the southern-most of the Japanese Home island--Kyūshū. The assauklt was scheduled for Novenber 1945. The island was mountainous, including Japan's most active volcano, Mt Aso. Kyūshū was the obvious objective because newly won bases in Okinawa could provide staging areas and tactical air cover. The whole island was not needed--only the southern area of the island. The Americans wanted enough of the island to build a comolex of air based that could support the invasion of the real prize--Honshu Next would be Operation Coronet in early-1946. This would involve the invasion of the Kantō Plain, near Tokyo, on the main Japanese island of Honshu. Honshu was the most populace Japanese island and where Tokyo was located. Honshu is the largest and most populous of the Home Islands. It was located south of Hokkaido across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyushu across the Kanmon Straits. It is the 7th largest island in the world, and the 2nd most populous after the Indonesian island of Java. Ove half of Japan's population was located on Honshu, mostly concentrated in the coastal areas and plains. It was this the historical center of Japanese cultural as well as the center of national political power. Hinshu was thus the primary invasion target. Airbases on southern Kyūshū captured in Operation Olympic would permit land-based air support for Operation Coronet.

Hokkido

Invading the northern island of Hokido was considered, but discarded, largely because of the inclemnet weather. American planners were initally talking an invasion of southeastern Hokkaido where it was believed that 10 divisions could seize the area needed for a complex of air bases on the Sapporo Plzin needed for tactical support in a battle for Honshu. Hokkaido was also seen as important to ensure American deliveries of supplies fot the Red Army invasion force in Manchuria. No one at the time envisioned a collapse of the Japanese Army in Manchuria within in days of a Soviet invasion. Hokkaido was also atttactive becuase of the extent of the buildup on Kyūshū. But given the importance of air power in American battle planning, the snow and inclemet weather argued against a northern invasion. As did the logistical train already established through the Central Pacific. When the time fram was reduced fromn1947-48 to November 1945, there simply was no time to buld another logistical train.

Kyūshū (Olympic--Novenber 1945))

The initial goal was thus to capture areas of the southern-most of the Japanese Home island--Kyūshū. The island was mountainous, including Japan's most active volcano, Mt Aso. Kyūshū was the obvious objective because newly won bases in Okinawa could provide staging areas and tactical air cover. The whole island was not needed--only the southern area of the island. The Americans wanted enough of the island to build a comolex of air based that could support the invasion of the real prize--Honshu

Honshu (Corinet--March 1946)

Next would be Operation Coronet in March 1946. This would involve the invasion of the Kantō Plain, near Tokyo, on the main Japanese island of Honshu. Honshu was the most populace Japanese island and where Tokyo was located. Honshu is the largest and most populous of the Home Islands. It was located south of Hokkaido across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyushu across the Kanmon Straits. It is the 7th largest island in the world, and the 2nd most populous after the Indonesian island of Java. One half of Japan's population was located on Honshu, mostly concentrated in the coastal areas and plains. It was this the historical center of Japanese cultural as well as the center of national political power. Hinshu was thus the primary invasion target. Airbases on southern Kyūshū captured in Operation Olympic would permit land-based air support for Operation Coronet.

Soviet Role


Soviet Goals

Stalin's goals in aggreeing to entering the war were primarily Manchuria and southern Sakalin. with the great victory in the West and the run up to Potsdam Conference, he began yto think about Japan itself, especially Hokkaido. Stalin asked his most respected military commander Marshal Georgy Zukov about the feasability of seizing Hokkaido. Zukov told his that four fieLd armies would be required which if redirected for such an 'adventurous move' would drastically weaken the now thourgly planned invasion of Manchuria. Zuhkov also raised the obvious questiion of how the men amd material needed would actually get to Hokkaido. The American efforts to build up a Soviet amphibious capability (Operation Hulu) was only beginning. Zhukov told Stalin that the needed naval component would not be ready for a year, if ever. Marshal Aleksandr Vasilevsky, Chief of the General Staff, agreed with Zhukov about the risks involved, describing itb as extremly dangerous and utterlky impractical. Molotov then injected that both Ameriuca and Britain would see this as a flagrant violatioin if the then still secret provisiins of the Yalta Agreement. As a resuly, Stalin approved only the vprviously envisioned opertion to invade Manchuria. souther Sakalin, and the Kurile Islands. (June 28). This was only 3 weeks before the Potsdam Conference. Hokkaido was left off the table.

Soviet invasion

At the same time that the Japanese were trying to get the Soviets to help negotizate an end to the War, the Soviet Union was completing their prepartions for a massive invasion, aided by special expanded American Lemd Lease shipments. The Soviets declared war (August 8). The Red Army invaded the following day (August 9). While the Red Army invasion was a massive success on land in Manchuria. Getting to Japan itself was a different matter. Operation Downfall was an American invasion plan, but Stalin wanted his part of Japan and had set his sites on Hokkaido in the north which the Americans had rejected as oart of Downfall. One part of Japan was easy prey--Sakalin. Japan had seized southern Sakalin during the Russo-Japanese War (1904-05). Here as the Soviets held northern Sakalin, this could be accomplished by a land invasion. Seizing Hokkaido, however, was much more formidable. The Soviets did not have a large navy and much of what they had was in the Atlantic. And they had virtully no landing craft, especially landing craft capsble of landing heavy weapons and suppliers in quantity.

Hokkaidio revisited

The huge success in Manchurua and the surrender of Japanese units without fighting to the death surprosed the Soviets. And the Japanese acceptance of the Potsdam Declaration, basically surrensring to the Americans, caused Stalin to rethink Hokkaido, raising the possibility of landing men an equipment wihout Japanese resistance as they were doing with the Americans. This would be useful in bargaining with the Americans over a joint occupation as was the case in Germany and perhaps a Soviet sector in Tokyo. Stalin cabled President Truman with a rather brusk demand for an occupation condstituting the northern half of Hokkaido saying that Soviet public opinion would be offended without this concession (August 15). This of course had not been discussed at Potsdam. Truman turned down the proposal flatly as contrary to the Yalta Agreement. As a result, orders were cut for Red Army's units of the 87th Rifle Corps (August 18) supported by naval and construcion batallions.. Units were to land and seize Rumoi, a small, remote port along the western coast of Hokkaido. The goal was to be in place by September 1, before the fornmal surrender ceremonies aboard the USS Missouri in Tokyo Bay. Rumoi was chosen because the Soviets needed a port to unload geavy equioment and it was far away from the Japanee divisions mostly defending the Sapoparo Plain in the Southeast where the Americans were likelky to land. And this would have to be accomplished without meaningful naval gunfire or tactical air support. Japanese units in the north (Hokkaido and the Kuriles) were prepared to resist the Soviets. The Soviets began with the Kurile Islands streaching northeast of Hokkido toward Kamchatka. These tiny islnds after theManchurian experience were believed to be easy targets. The Soviet 87th Rifle Corps invaded the northern island of Shumushu close to the tip of the Kamachatka Peninsula (August 18). The island was heavily garisoned, largely because the Jaosnese believed it was an American invasion target. The Soviets used the American LCI assault landing craft and lost most of them from Japanese shore bandings. And then put up a fierce resistance. The Emperor demanded the garrisons on the islands to surrender as part of the general surrender (August 23). Some units continued to resist. The unexpected resistance on these tiny islands was sobering. Not so mucvh bercause of the loss of life, that was not a concern for Stalin, but because he did not want a defeat to tarnish the image of Soviet power. [Giangreco, Hell.] He thus cancelled the planned Hokkaido 'escapade' (Zukov's descriptionn which Stalin probably did not appreciate) where the Soviets would have faced a much more substantial Japanese force. The American would eventually oversee the demobilizatiion of some 292,000 well armed Japanese soldiers with air support on Hokkaido. [Giangreco, Hell.] Making in triple the size of the Japanese garrison on Okinawa which the Soviets would have to face without naval gun support or the massive American air umbrella. And this did not include the Japanese People's Volunteer Corps as well as the police force. In addition, the Japanese had built up their Hokkaido air defenses in anticipation of a possible American invasion. Another imprtant historians goes into detail about how the Soviet milkitary just did not have the military capability of seizing Hokkaido. [Glantz] Had the Japanese not surendered when they did and Downfall would have been necessary, the Soviets would have had time to prepare an invasion of Hokkaido. Abnd we would today have a Japanese regime rather like the Orwellin Kim regime in North Korea.

British Component

The British played a role Downfall, amely the British Asian Fleet. Even before VE Day, the Royal Navy began sending fleet elements east to support first operations in South East Asia and then operations against Japan itself. The Royal Navy was not needed in naval operations. The Imperial Navy no longer existed as an effectgive fightung force. What was left of it had been kargely destroyed in the climatic Battle of Leyte Gulf (Ocoiber 1944). There were also plans to redeploy Commonwealth forces for the invasion of Japan. The Americans were not all that interestec at Quebec, but after the firece fighting for Iwo Jima and Okinawa, they became more interested. [Weinberg, p. 844.] The U.S. Pacigic Fleet needed all the help it coud get to cover the invasion fleet from the subsrantual secret airforce of Kamakazee planes that Japan had built up in preopartiin for the invasion.

Japanese Preparations

The Japanese easily predicted the principal target. All they had to do was to calculate the range of air cover from Okinawa. As a result, the Japanese Army began to heavily reinforce Kyūshū. There was sone strebthening of defenses in the north (Hokkaido and the Kuriles), but the najor effort was on Kyūshū. In addition, the Japanese had amassed large numbers of aircraft which could be used in Kamakazi attacks on any invasion fleet. Many Japanese today maintain that Japan was near defeat and ready to surrender. It is certainly true that the military situation for Japan was hopeless and the population approaching starvation. This does not mean, however, that the Imperial Army was preparing to surrender. The situation had been hopless in every island fight after vGuadakcanl and the Japanese still fought bto the death. Ultra intercepts reported that the Japanese were strenthening their defenses in southern Kyushu. The Army High Command deployed three divisions there (June 1945) and that had been increased to 9 divisions (July 1945). Reinforcenents were brought back from China and Manchuria. In addition, the Army had incouraged, even forced, civilians to commit suiside as on Siapan and Okinawa.

Naval Support

The naval support needed for the invasion taxed even the enormous resources of the U.S. Pacific Fleet. They included 18 fleet (mostky Essex class) carriers, the two Midway xkass carries, 7 light carriers, 19 battleships and battle cru=isers, 61 cruisers, 63 escort carriers, 315 destroyers and destroyer escorts, and a huge contingent of minesweepers and assaukt vessels. This was karger than the D-Day battle group, but of couse air bases in England were available fir the Nirmandy landings.,

Air Support

Air support was central in American military operations. Gen. Marshal advocated an invasion and Adm. Nimitz supported him. Gen. MacArthur was assigbned to command the invasion and began the planning. His plan provided that once Kyushu was secured, that air bases there could cover more northerly landings close to Tokyo.

Dimensions

Had Downfall been launched, it would have been the largest amphibious operation in history, dwarfing the D-Day landings which had the advantage of being just a few miles across the Channel.

Terraine

In addition to the huge logistical issues, Japanese geography and terraine made these invasions a daunting prospect. Japan is a very mountanouss country. There were few flat plains making sweeping armored thursts possible. The rugged terraine reduced the overwhealming Americam advantage in firepower.

Estimated Casualties

The terraine mandated that the fight for the Home Islands woukd be more like Okinawa, meaning huge American casualties and much larger Japanese losses, in both military personnel and civilians. And this does noyt includeg the fact that Japanese civilans were already approachiung starvation. The Japanese were palnaning a massive defense of Kyūshū. American casualty estimates ran up into the millions. The Japanese casualties which had been 10 to 1 in just military personnel on Okinawa would have been astronomical on the Home Island. And the Japanese as part of Operation Ketsugo planed to use civilians to resist the invasion. Some observers are convinced that Japanese civilian casualties in any invasion of the Home Islands would have been the greatest civilian disaster of World War II. Millions of Japanese soldiers and civilians may have died if America had invaded Japan. Ultra intercepts revealed constant strengthening of forces on Kyushu which reached 13 divisions.

American Expectaions

The American military fully expected, and had every reaon to do so, that the Japanese Army would resist to the death just had they had done on Okinawa. Both Marshall and Nimitz began to have second thoughts. Planners began to consider alternatives like poison gas, atomic bombs, or changing the invasion site. MacArthur continued to insist on Kyushu. [Giangreco, Hell]

Sources

Giangreco, D.M. Hell to Pay: Operation Downfall and the Invasion of Japan, 1945-1947

Giangreco, D.M. Talk: WWII--Strategies for the invasion & Defense of Japan (Institute for the Study of Strategy and Politics) C-Span broadcast August 6, 2015. The talk was partnof a symposium entitled "Endgame: August 1945 in Asia and the Pacific".

Glantz, David. August Storm: The Soviet 1945 Strategic Offensive in Manchuria (Portland, Oregon: Frank Cass Publishers, 2004).

Weinberg, Gerhard L. A World at Arms: A Global History of World War II (Cambrige Universit Press: New York, 2005), 1178p.






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Created: 5:54 PM 11/6/2020
Last updated: 4:15 AM 11/10/2020