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Operation Downfall

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Operation Downfall was the overall Allied plan for the invasion of Japan at the end of World War II. It was scheduled to occur in two parts: Operation Olympic, the invasion of Kyushu, set to begin in November 1945; and later Operation Coronet, the invasion of Honshu near Tokyo, scheduled for the spring of 1946. Following the atomic bombing of Hiroshima and Nagasaki, and the Soviet declaration of war against Japan, the Japanese surrendered and the operation was cancelled.

Contents

Planning for Downfall

Responsibility for planning the operation fell to the US commanders: Fleet Admiral Chester Nimitz, General of the Army Douglas MacArthur and the Joint Chiefs of Staff—Fleet Admirals Ernest King and William Leahy, and Generals of the Army George Marshall and Hap Arnold—who controlled the Twentieth Air Force, the strategic bombers. At the time, the development of the atomic bomb was a very closely guarded secret, known only to a few top officials outside the Manhattan Project, and planning for the invasion did not take its existence into consideration.

Throughout the Pacific War, unlike the European theatre, the Allies were unable to agree on a single Commander-in-Chief (C-in-C) in the Pacific. Allied command was divided into regions: by 1945, for example, Chester Nimitz was Allied C-in-C Pacific Ocean Areas, while Douglas MacArthur was Supreme Allied Commander, South West Pacific Area. For an invasion of Japan, a unified command was deemed necessary. Inter-service squabbling over who it should be—the US Navy wanted Nimitz, while the US Army wanted MacArthur—was so serious that it threatened to derail planning. Ultimately, the navy partially conceded and MacArthur was to have total command of all forces, if circumstances made it necessary.

Considerations

The primary considerations that the planners had to deal with were time and casualties—how could they force Japan's surrender as quickly as possible, with as few Allied casualties as possible? In 1943, the Joint Chiefs agreed that Japan should be forced to surrender not more than one year after Germany's surrender. They were inspired to do this after seeing British-made plans that did not call for invasion of the home islands until the fall of 1947. Prolonging the war to such an extent was considered dangerous for national morale.

The U.S. Navy urged the use of blockade and airpower to bring about Japan's capitulation. They proposed operations to capture airbases in nearby Shanghai, China, and in Korea. These locations would give the U.S. Army Air Force a series of forward airbases from which to operate against Japan. The U.S. Army, on the other hand, argued that such a strategy could prolong the war indefinitely and expend lives needlessly, and therefore that an invasion was necessary. They supported mounting a large-scale thrust directly against the Japanese homeland, with none of the side operations that the Navy had suggested. Ultimately, the Army's viewpoint won out.

Physically, Japan made an imposing target, having few beaches suitable for invasion. Only Kyushu (the southernmost island of Japan) and the beaches of the Kanto plain (both southwest and southeast of Tokyo) made suitable invasion zones. The allies decided to launch a two-stage invasion. Operation Olympic would attack southern Kyushu. Airbases would be established, and those would give cover for Operation Coronet, the attack on Tokyo Bay.

Assumptions

While the geography of Japan was fixed, the planners could only estimate the defending forces they would face. Based on intelligence available early in 1945, their assumptions included:

  • "That operations in this area will be opposed not only by the available organized military forces of the Empire, but also by a fanatically hostile population."
  • "That approximately three (3) hostile divisions will be disposed in Southern KYUSHU and an additional three (3) in Northern KYUSHU at initiation of the OLYMPIC operation."
  • "That total hostile forces committed against KYUSHU operations will not exceed eight (8) to ten (10) divisions and that this level will be speedily attained."
  • "That approximately twenty-one (21) hostile divisions, including depot divisions, will be on HONSHU at initiation of that operation [Coronet] and that fourteen (14) of these divisions may be employed in the KANTO PLAIN area."
  • "That the enemy may withdraw his land-based air forces to the Asiatic Mainland for protection from our neutralizing attacks. That under such circumstances he can possibly amass from 2,000 to 2,500 planes in that area by exercise of rigid economy, and that this force can operate against KYUSHU landings by staging through homeland fields."

Olympic

Missing image
Operation_Olympic.jpg
Operation Olympic was to attack southern Japan.

Operation Olympic, the invasion of Kyushu, was to begin on "X-Day", which was scheduled for November 1, 1945. The combined Allied naval armada would have been the largest ever assembled, including forty-two aircraft carriers, twenty-four battleships and almost four hundred destroyers and destroyer escorts. Fourteen US divisions were scheduled to take part in the initial landings. Using Okinawa as a staging base, Olympic was to seize the southern portion of Kyushu, to use as a staging point. It was also to include a deception plan, known as Operation Pastel.

Before the main invasion, the offshore islands of Tanegashima, Yakushima, and the Koshiki Retto were to be taken, starting on X-5. The Okinawa experience had proved the value of having secure anchorages close at hand, for ships not needed off the landing beaches and for ships damaged by air attack.

Kyushu was to be invaded by U.S. Sixth Army at three points—Miyazaki, Ariake, and Kushikino. (If Kyushu were a clock, these would roughly correspond to 4, 5, and 7 o'clock, respectively.) The 35 landing beaches were all named for automobiles: Austin, Buick, Cadillac, ..., Stutz, Winton, and Zephyr. With one corps assigned to each landing, the invasion planners assumed that the Americans would outnumber the Japanese by roughly three to one. In early 1945 Miyazaki was virtually undefended, while Ariake (with its nearby good harbor) was heavily defended. Although weakly defended, the imposing terrain at Kushikino meant that the Marines who landed there would probably have the toughest time.

The invasion was not supposed to conquer the entire island, just the southernmost third of it (indicated by the dashed line on the map, "general limit of northern advance"). Southern Kyushu would become a staging ground for Operation Coronet and would give the Allies a valuable airbase from which to operate.

(See Order of Battle for Olympic.)

Missing image
Operation_Coronet.jpg
Operation Coronet was planned to take Tokyo.

Coronet

Operation Coronet, the invasion of Honshu at the Tokyo Plain south of the capital, was to begin on "Y-Day", which was scheduled for March 1, 1946. Coronet would have been the largest amphibious operation of all time, with 25 divisions (including the floating reserve) earmarked for the initial operations. U.S. First Army would have invaded at Kujukuri Beach, on the Boso Peninsula, while U.S. Eighth Army invaded at Hiratsuka, on Sagami Bay. Both armies would then drive north and inland, meeting at Tokyo.

(See Order of Battle for Coronet.)

Redeployment

Olympic was to be mounted with resources already in the Pacific. These included the British Pacific Fleet, which was actually a Commonwealth formation, and included at least a dozen aircraft carriers and several battleships. The 20 squadron-strong Australian First Tactical Air Force took part in the invasion of the Philippines in October 1944, and it is therefore probable that it would have augmented US close air support units over Japan. The only major re-deployment for Olympic was "Tiger Force", the name given to a group of more than 30 Commonwealth bomber squadrons, scheduled to be transferred from RAF Bomber Command in Europe to airbases on Okinawa.

If reinforcements had been needed, they could have been provided out of the forces being assembled for Coronet, which would have needed the redeployment of substantial Allied forces from Europe, South Asia, Australasia and elsewhere. These would have included the U.S. First Army (fifteen divisions) and Eighth Air Force, which were in Europe. The redeployment was complicated by the simultaneous partial demobilization of the U.S. Army, which drastically reduced the divisions' combat effectiveness, by stripping them of their most experienced officers and men.

A Commonwealth Corps, initially made up of infantry divisions from the British Army, Australian Army and Canadian Army was being formed for use in Coronet. Substantial reinforcements would have been available from those countries, as well as other parts of the Commonwealth, although MacArthur had rejected proposals to include an Indian Army division, due supposedly to language issues. He was also pushing for the Commonwealth Corps to be organised along the lines of a US Army corps, and to use predominantly US equipment, to smooth out supply and logistics. A British officer, Lt. Gen. Sir Charles Keightley, had been nominated to lead the corps.

Ketsu-Go

Meanwhile, the Japanese had their own plans. Initially they were concerned about an invasion during the summer of 1945. However, the Battle of Okinawa went on so long that they concluded the Allies would not be able to launch another operation before the typhoon season, during which the weather would be too risky for amphibious operations. Japanese intelligence predicted fairly closely where the invasion would take place: southern Kyushu at Miyazaki, Ariake Bay, and/or the Satsuma Peninsula. While Japan no longer had a realistic prospect of winning the war, it could perhaps raise the cost of conquering Japan too high for the Allies to accept, leading to some sort of armistice. The Japanese plan for defeating the invasion was called Ketsu-Go, "Decisive Operation".

Kamikazes

The Japanese defense relied heavily on kamikaze planes. In addition to fighters and bombers, they reassigned almost all of their trainers for the mission, trying to make up in quantity what they lacked in quality. Between them, the Army and Navy had more than 10,000 aircraft ready for use in July, and would have had somewhat more by October—and were planning to use almost all that could reach the invasion fleets.

During the Battle of Okinawa, less than 2,000 kamikazes had gotten about one hit per nine planes that made an attack. At Kyushu, given the more favorable circumstances, they hoped to get one for six. The Japanese estimated that the planes would sink more than 400 ships, and since they were training the pilots to target transports rather than carriers and destroyers, the casualties would be disproportionately greater than at Okinawa. One staff study estimated that the kamikazes could destroy a third to a half of the invasion force before its landings.

Naval forces

The Imperial Japanese Navy no longer had any ships available larger than destroyers. By August they did have about 100 Koryu-class midget submarines, 250 smaller Kairyu-class midget submarines, and 1,000 Kaiten manned torpedoes. The Japanese Army had 800 Shinyo suicide boats.

Ground forces

In any amphibious operation, the defender has two choices for defensive strategy—strong defense of the beaches, or defense in depth. Early in the war (such as at Tarawa) the Japanese employed strong defenses on the beaches themselves, with little or no manpower in reserve. This tactic proved to be very vulnerable to pre-invasion shore bombardment. Later in the war, at Peleliu, Iwo Jima, and Okinawa, the Japanese switched strategy and dug in their forces in the most defensible terrain. Fighting degenerated into long battles of attrition, with very high American casualties, but no hope of victory for the Japanese.

For the defense of Kyushu, the Japanese took an intermediate posture, with the bulk of their defensive forces a few km inland from the shore—back far enough to not be completely exposed to naval gunnery, but close enough that the Americans could not establish a secure foothold before engaging them. The counteroffensive forces were still further back, prepared to move against whichever landing seemed to be the main effort.

In March 1945, there was only one combat division in Kyushu. Over the next four months the Japanese Army transferred forces from Manchuria, Korea, and northern Japan, while raising other forces in place. By August, they had fourteen divisions and various smaller formations, including three tank brigades, for a total of 900,000 men.

The Japanese were able to raise large numbers of new soldiers, but equipping them was harder. By August, the Japanese Army had the equivalent of 65 divisions in the homeland, but only enough equipment for 40, and only enough ammunition for 30. The Japanese did not formally decide to stake everything on the outcome of the Battle of Kyushu, but they concentrated their assets to such a degree that there would be little left in reserve. By one estimate, the forces in Kyushu had 40% of all the ammunition in the Home Islands.

In addition, the Japanese had organized nearly all adult civilians into the Patriotic Citizens Fighting Corps to perform combat support, and ultimately combat jobs. Weapons and training were generally lacking, but they were expected to make do with what they had.

One mobilized high school girl, Yukiko Kasai, found herself issued an awl and told, "Even killing one American soldier will do. ... You must aim for the abdomen." (Richard B. Frank, Downfall)

Allied reevaluation of Olympic

Air threat

U.S. military intelligence initially estimated the number of Japanese aircraft to be around 2,500. The Okinawa experience was bad—almost two fatalities and a similar number wounded per sortie—and Kyushu was likely to be worse. To attack the ships off Okinawa, planes had to fly long distances over open water; to attack the ships off Kyushu, they could fly overland and then short distances out to the landing fleets. Gradually, intelligence learned that the Japanese were devoting all their aircraft to the kamikaze mission, and taking effective measures to conserve them until the battle. An Army estimate in May was 3,391 planes; in June, 4,862; in August, 5,911. A Navy estimate, abandoning any distinction between training and combat aircraft, in July was 8,750; in August, 10,290.

The Allies made preparations, adding more fighter squadrons to the carriers in place of torpedo- and dive-bombers, and converting B-17s into airborne radar pickets—the ancestors of the AWACS. Adm. Nimitz came up with a plan for a pre-invasion feint, sending a fleet to the invasion beaches a couple of weeks before the real invasion, to lure out the Japanese on their one-way flights, who would then find—instead of the valuable, vulnerable transports—ships loaded with antiaircraft guns from stem to stern.

Ground threat

Through April, May, and June, Allied intelligence followed the buildup of Japanese ground forces, including five divisions added to Kyushu, with great interest but some complacency, still projecting that in November the total for Kyushu would be about 350,000 servicemen. That changed in July, with the discovery of four new divisions and indications of more to come. By August, the count was up to 600,000, and Magic had identified nine divisions in southern Kyushu—three times the expected number. (And in fact this was still a serious underestimate of Japanese strength; see above.)

(Compare the estimate made in early July (http://www.odci.gov/csi/monograph/4253605299/pg16.gif) with the estimate made in early August (http://www.odci.gov/csi/monograph/4253605299/pg18.gif).)

The intelligence revelations about Japanese preparations on Kyushu emerging in mid-July transmitted powerful shock waves both in the Pacific and in Washington. On 29 July, [MacArthur's intelligence chief, Maj. Gen. Charles A.] Willoughby ... noted first that the April estimate allowed for the Japanese capability to deploy six divisions on Kyushu, with the potential to deploy ten. "These [six] divisions have since made their appearance, as predicted," he observed, "and the end is not in sight." If not checked, this threatened "to grow to [the] point where we attack on a ratio of one (1) to one (1) which is not the recipe for victory." (Richard B. Frank, Downfall)

The buildup of Japanese troops on Kyushu led American war planners, most importantly Gen. George Marshall, to consider drastic changes to Olympic, or replacing it with a different plan for invasion.

Chemical weapons

Because of its predictable wind patterns and several other factors, Japan was particularly vulnerable to gas attack. Such attacks would neutralize the Japanese tendency to fight from caves—caves would only increase the soldiers' exposure to gas.

Although chemical warfare had been outlawed by the Geneva Protocol, neither the United States nor Japan were signatories at the time. While the United States had promised never to initiate gas warfare, earlier in the war Japan had used gas against the Chinese in Manchuria (see Unit 731). This gave the United States a reason for their use, and as John Ray Skates notes,

"Fear of Japanese retaliation [to chemical weapon use] lessened because by the end of the war Japan's ability to deliver gas by air or long-range guns had all but disappeared. In 1944 Ultra revealed that the Japanese doubted their ability to retaliate against United States use of gas. 'Every precaution must be taken not to give the enemy cause for a pretext to use gas,' the commanders were warned. So fearful were the Japanese leaders that they planned to ignore isolated tactical use of gas in the home islands by the US forces because they feared escalation." (John Ray Skates, The Invasion of Japan, ISBN 0-87249-972-3)

Atomic weapons

On Marshall's orders, Maj. Gen. John Hull looked into the tactical use of atomic weapons. He reported that perhaps seven bombs would be available by X-Day, which could be dropped on defending forces. Col. Lyle E. Seeman advised that American troops not enter an area hit by a bomb for "at least 48 hours"; the risk of fallout was not well-understood.

Alternative targets

The Joint Staff Planners, taking note of the extent to which the Japanese had concentrated on Kyushu at the expense of the rest of Japan, considered alternate places to invade, including the island of Shikoku, or northern Honshu at Sendai or Ominato—or skipping the preliminary invasion and going directly at Tokyo. Attacking northern Honshu would have the advantage of a much weaker defense, but the cost of giving up air support from Okinawa, except the B-29s.

Prospects for Olympic

Gen. Douglas MacArthur dismissed any need to change his plans. "I am certain that the Japanese air potential reported to you as accumulating to counter our OLYMPIC operation is greatly exaggerated. ... As to the movement of ground forces, ... I do not credit, however, the heavy strengths reported to you in southern Kyushu. ... In my opinion, there should not be the slightest thought of changing the OLYMPIC operation." However Adm. Ernest King, the CNO, was prepared to officially oppose proceeding with the invasion, with Adm. Nimitz's concurrence, which would have set off a major dispute within the United States government.

At this juncture, the key interaction would likely have been between Marshall and Truman. There is strong evidence that Marshall remained committed to an invasion as late as 15 August. ... But tempering Marshall's personal commitment to invasion would have been his comprehension that civilian sanction in general, and Truman's in particular, was unlikely for a costly invasion that no longer enjoyed consensus support from the armed services. (Richard B. Frank, Downfall)

Unbeknowst to the Americans, the Soviets were preparing to follow up their invasions of Sakhalin and the Kuril Islands with an invasion of the weakly-defended island of Hokkaido by the end of August, which would have put pressure on the Allies to do something sooner than November. On August 15, the Japanese agreed to surrender, rendering the whole question of invasion moot.

Estimated casualties for Downfall

Given the Japanese predilection for fanatical resistance, the fact that Japanese civilians were being encouraged to become suicide attackers, and the large number of Japanese troops to be faced, high casualties were seen to be inevitable, but nobody knew with certainty how high. Several people made estimates but they varied widely in numbers, assumptions, and purposes—which included advocating for and against the invasion. (Afterwards, they were reused to argue for and against the atomic bombings of Hiroshima and Nagasaki.)

Everybody based their estimates on the experience of the preceding campaigns, but they could draw different lessons:

In a study done by the Joint Chiefs of Staff in April, the figures of 7.45 casualties/1000 man-days and 1.78 fatalities/1000 man-days were developed. This implied that a 90-day Olympic campaign would cost 456,000 casualties, including 109,000 dead or missing. If Coronet took another 90 days, the combined cost would be 1,200,000 casualties, with 267,000 fatalities.

A study done by Adm. Nimitz's staff in May estimated 49,000 casualties in the first 30 days, including 5,000 at sea. A study done by Gen. MacArthur's staff in June estimated 23,000 in the first 30 days and 125,000 after 120 days. When these figures were questioned by Gen. Marshall, MacArthur submitted a revised estimate of 105,000, in part by deducting wounded men able to return to duty.

In a conference with President Truman on 18 June, Marshall, taking Luzon as the best model for Olympic, thought the Americans would suffer 31,000 casualties in the first 30 days (and ultimately 20% of Japanese casualties, which implied a total of 70,000 casualties). Adm. Leahy, more impressed by Okinawa, thought the American forces would suffer a 35% casualty rate (implying an ultimate toll of 268,000). Adm. King thought that casualties in the first 30 days would fall between Luzon and Okinawa, i.e., between 31,000 and 41,000.

Of these estimates, only Nimitz's included losses of the forces at sea, though in the Battle of Okinawa kamikazes had inflicted 1.78 fatalities/sortie, and the troop transports off Kyushu would be much more exposed. Moreover, all these estimates were done using intelligence that grossly underestimated Japanese strength being gathered for the battle of Kyushu in numbers of soldiers and kamikazes, by factors of at least three.

A study done for Secretary of War Henry Stimson's staff by William Shockley estimated that conquering Japan would cost 1.7–4 million American casualties, including 400,000–800,000 fatalities, and five to ten million Japanese fatalities. The key assumption was large-scale participation by civilians in the defense of Japan.

Outside the government, well-informed civilians were also making guesses. Kyle Palmer, war correspondent for the Los Angeles Times, said half a million to a million Americans would die by the end of the war. Herbert Hoover, in memorandums submitted to Truman and Stimson, also estimated 500,000–1,000,000 fatalities, and were believed to be conservative estimates. (But it is not known if Hoover discussed these specific figures in his meetings with Truman. The chief of the Army Operations division thought them "entirely too high" under "our present plan of campaign.")

For context, the Battle of Normandy had cost 63,000 casualties in the first 48 days. The entire war cost the United States a total of just over a million casualties, with 300,000 fatalities.

Nearly 500,000 Purple Heart medals were manufactured in anticipation of the casualties resulting from the invasion of Japan. As of 2005, all the American military casualties of the following sixty years—including the Korean and Vietnam Wars—have not exhausted that stockpile. [1] (http://hnn.us/articles/1801.html)

References

Alternate history

  • Westheimer, David, Lighter than a Feather. Boston: Little, Brown & Co., 1971. (Olympic)
  • Coppel, Alfred, The Burning Mountain. Harcourt Brace & Co, 1983. (Coronet)

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