Editing Operation Downfall (section)

== Allied re-evaluation of Operation Olympic ==

===Air threat===
US military intelligence initially estimated the number of Japanese aircraft to be around 2,500.{{sfn|Frank|p=206}} The Okinawa experience was bad for the US—almost two fatalities and a similar number wounded per [[sortie]]—and Kyūshū was likely to be worse. To attack the ships off Okinawa, Japanese planes had to fly long distances over open water; to attack the ships off Kyūshū, 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 July Navy estimate, abandoning any distinction between training and combat aircraft, was 8,750; in August, 10,290.{{sfn|Frank|pp= 209–10}} By the time the war ended, the Japanese actually possessed some 12,700 aircraft in the Home Islands, roughly half ''kamikazes''.{{sfn|Giangreco|2009|p=xviii}} ''Ketsu'' plans for Kyushu envisioned committing nearly 9,000 aircraft according to the following sequence:<ref>JM-85 pp. 18–21</ref>

* 140 reconnaissance planes to detect the approach of the Allied fleet.
* 330 Navy bombers flown by highly trained pilots to attack the Allied carrier task force to prevent it from supporting the invasion convoys.
* 50 "land attack planes," 50 seaplane bombers, and 50 torpedo bombers flown by highly trained pilots for night attacks on convoy escorts.
* 825 Navy ''kamikazes'' to attack the landing convoys prior to their arrival off Kyūshū.
* 2,500 Army aircraft (conventional as well as suicide), together with 2,900 Naval trainers for ''kamikaze'' attacks against the landing fleet as it arrived and anchored (5,400 total).
* 2,000 Army and Navy "air superiority" fighters to escort the ''kamikazes'' and strafe landing ships.
* 100 transport planes carrying 1,200 commandos for a raid on the US airbases on Okinawa, following the success of earlier smaller-scale operations.

The Japanese planned to commit the majority of their air forces to action within 10 days after the Allied fleet's arrival off Kyūshū. They hoped that at least 15 to 20% (or even up to a half) of the US transport ships would be destroyed before disembarkation.<ref name="OLYMPIC VS KETSU-GO">{{Cite web |title=OLYMPIC VS KETSU-GO |url=http://www.ibiblio.org/pha/war.term/olympic.html |access-date=February 24, 2023 |website=www.ibiblio.org}}</ref> The United States Strategic Bombing Survey subsequently estimated that if the Japanese managed 5,000 ''kamikaze'' sorties, they could have sunk around 90 ships and damaged another 900, roughly triple the Navy's losses at Okinawa.<ref name=":0">{{Cite web |title=USSBS Report 62, Military Analysis Division, Japanese Air Power {{!}} PDF p. 25|url=https://www.scribd.com/document/50311346/USSBS-Report-62-Military-Analysis-Division-Japanese-Air-Power |access-date=February 24, 2023 |website=Scribd |language=en}}</ref>

Allied counter-''kamikaze'' preparations were known as the [[Big Blue Blanket]]. This involved adding more fighter squadrons to the carriers in place of [[Torpedo bomber|torpedo]] and [[dive bomber]]s, and converting [[Boeing B-17 Flying Fortress|B-17s]] into airborne [[radar picket]]s in a manner similar to present-day [[Airborne Early Warning and Control|AWACS]]. Nimitz planned 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 ships bristling with anti-aircraft guns instead of the valuable, vulnerable transports.{{citation needed|date = July 2014}}

The main defense against Japanese air attacks would have come from the massive fighter forces being assembled in the [[Ryukyu Islands]]. The US Army Fifth and Seventh Air Forces and US Marine air units had moved into the islands immediately after the invasion, and air strength had been increasing in preparation for the all-out assault on Japan. In preparation for the invasion, an air campaign against Japanese airfields and transportation arteries had commenced before the Japanese surrender.{{citation needed|date = July 2014}}

===Ground threat===
Through April, May, and June, Allied intelligence followed the buildup of Japanese ground forces, including five divisions added to Kyūshū, with great interest, but also some complacency, still projecting that in November the total for Kyūshū 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 (cryptography)|Magic cryptanalysis]] had identified nine divisions in southern Kyūshū—three times the expected number and still a serious underestimate of the actual Japanese strength.

Estimated troop strength in early July was 350,000,{{r|MacEachin p16}} rising to 545,000 in early August.{{r|MacEachin p18}}

{{blockquote|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, [[Charles A. Willoughby|Major General Charles A. Willoughby]], was the first to note 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."{{sfn|Frank|p= 211|loc= Willoughby's Amendment 1 to "G-2 Estimate of the Enemy Situation with Respect to Kyushu"}}}}

By the time of surrender, the Japanese had over 735,000 military personnel either in position or in various stages of deployment on Kyushu alone.{{sfn|Giangreco|2009|loc=Appendix B.}} The total strength of the Japanese military in the Home Islands amounted to 4,335,500, of whom 2,372,700 were in the Army and 1,962,800 in the Navy.<ref>[http://www.geocities.co.jp/WallStreet/2687/siryo/siryo16.html Ministry of Health and Welfare, 1964.] {{Webarchive|url=https://web.archive.org/web/20160105112919/http://www.geocities.co.jp/WallStreet/2687/siryo/siryo16.html |date=January 5, 2016 }} Retrieved July 21, 2015.</ref> The buildup of Japanese troops on Kyūshū led American war planners, most importantly General George Marshall, to consider drastic changes to Olympic, or replacing it with a different invasion plan.{{citation needed|date=October 2009}}

===Chemical weapons===
Fears of "an Okinawa from one end of Japan to the other"<ref name="Burnham1995">{{cite news|last=Burnham|first=Alexander|date=July 1, 1995|title=Okinawa, Harry Truman, and the Atomic Bomb|volume=71|work=A National Journal of Literature & Discussion|publisher=VQR|number=#3|url=http://www.vqronline.org/essay/okinawa-harry-truman-and-atomic-bomb|access-date=March 17, 2017}}</ref> encouraged the Allies to consider unconventional weapons, including chemical warfare. Widespread [[chemical warfare]] was considered against Japan's population<ref>{{Cite web|url=https://www.opendemocracy.net/conflict/byanymeansnecessary_2727.jsp|title=By any means necessary: the United States and Japan|last=Rogers|first=Paul|date=August 4, 2005|website=openDemocracy|access-date=July 17, 2018}}</ref> and food crops.<ref>{{cite news |last=Walsh |first=Liam |date=December 7, 2011 |title=World War II plan to poison Japanese crops revealed |url=http://m.couriermail.com.au/news/national/world-war-ii-poison-plan-revealed/story-e6freooo-1226215619956 |newspaper=the Courier-Mail |location=Australia |access-date=October 10, 2015 }}{{Dead link|date=October 2022 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> While large quantities of gas munitions were manufactured and plans were drawn, it is unlikely they would have been used. [[Richard B. Frank]] states that when the proposal reached Truman in June 1945, he vetoed the use of chemical weapons against personnel; their use against crops, however, remained under consideration. According to [[Edward J. Drea]], the strategic use of chemical weapons on a massive scale was not seriously studied or proposed by any senior American leader; rather, they debated the ''tactical'' use of chemical weapons against pockets of Japanese resistance.<ref>{{cite web |url=https://www.pbs.org/wgbh/americanexperience/features/online-forum/ |title=Victory in the Pacific Online Forum|date=May 6, 2005 |website=[[PBS]] |access-date=October 21, 2016 }}</ref>

Although chemical warfare had been outlawed by the [[Geneva Protocol]], neither the United States nor Japan was a signatory at the time. While the US had promised never to initiate gas warfare, Japan [[Japanese war crimes#Use of chemical weapons|had used gas against the Chinese]] earlier in the war:{{sfn|Skates|p=84}}

{{blockquote|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 (cryptography)|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.{{sfn|Skates|p= 97}}|Skates}}

In addition to use against people, the U.S. military considered chemical attacks to kill crops in an attempt to starve the Japanese into submission. The Army began experimenting with compounds to destroy crops in April 1944, and within one year had narrowed over 1,000 agents to nine promising ones containing [[Phenoxy herbicide|phenoxyacetic acids]]. One compound designated LN-8 performed best in tests and went into mass production. Dropping or spraying the [[herbicide]] was deemed most effective; a July 1945 test from an SPD Mark 2 bomb, originally crafted to hold biological weapons like [[anthrax]] or [[ricin]], had the shell burst open in the air to scatter the chemical agent. By the time the war ended, the Army was still trying to determine the optimal dispersal height to cover a wide enough area. The ingredients in LN-8 and another tested compound would later be used to create [[Agent Orange]], used during the [[Vietnam War]].<ref>{{cite web |url=https://warisboring.com/the-pentagon-nearly-attacked-japan-with-chemical-weapons-in-1945-b4494e76884f#.b2wy13xdn |title=America Nearly Attacked Japan With Chemical Weapons in 1945 |last1=Trevithick |first1=Joseph |date=June 10, 2016 |website=War is Boring.com |access-date=June 11, 2016 |url-status=dead |archive-url=https://web.archive.org/web/20160907125639/https://warisboring.com/the-pentagon-nearly-attacked-japan-with-chemical-weapons-in-1945-b4494e76884f#.b2wy13xdn |archive-date=September 7, 2016 }}</ref>

===Nuclear weapons===
On Marshall's orders, Major General [[John E. Hull]] looked into the [[Tactical nuclear weapon|tactical use of nuclear weapons]] for the invasion of the Japanese home islands, even after the dropping of two [[Strategic nuclear weapons|strategic atomic bombs]] on Japan (Marshall did not think that the Japanese would capitulate immediately). Colonel Lyle E. Seeman reported that at least seven [[Fat Man]]-type plutonium implosion bombs would be available by X-Day, which could be dropped on defending forces. Seeman advised that American troops not enter an area hit by a bomb for "at least 48 hours"; the risk of [[nuclear fallout]] was not well understood, and such a short time after detonation would have exposed American troops to substantial radiation.{{sfn|Frank|pp= 312–13}}

[[Ken Nichols]], the District Engineer of the [[Manhattan Engineer District]], wrote that at the beginning of August 1945, "[p]lanning for the invasion of the main Japanese home islands had reached its final stages, and if the landings actually took place, we might supply about fifteen atomic bombs to support the troops."{{sfn|Nichols|p=201}} An air burst {{convert|1800|-|2000|ft|m|abbr=on}} above the ground had been chosen for the (Hiroshima) bomb to achieve maximum blast effects, and to minimize residual radiation on the ground, as it was hoped that American troops would soon occupy the city.{{sfn|Nichols|pp=175, 198, 223}}

===Alternative targets===
The Joint Staff planners, taking note of the extent to which the Japanese had concentrated on Kyūshū at the expense of the rest of Japan, considered alternate places to invade such as the island of [[Shikoku]], northern Honshu at [[Sendai, Miyagi|Sendai]], or [[Ominato, Aomori|Ominato]]. They also considered skipping the preliminary invasion and going directly at Tokyo.{{sfn|Frank|pp=273–74}} Attacking northern Honshu would have the advantage of a much weaker defense but had the disadvantage of giving up land-based air support (except the [[Boeing B-29 Superfortress|B-29s]]) from [[Okinawa Prefecture|Okinawa]].{{citation needed|date = July 2014}}

===Prospects for Olympic===
MacArthur dismissed any need to change his plans:

{{blockquote|I am certain that the Japanese air potential reported to you as accumulating to counter our OLYMPIC operation is greatly exaggerated.&nbsp;… As to the movement of ground forces… I do not credit… the heavy strengths reported to you in southern Kyushu.&nbsp;… In my opinion, there should not be the slightest thought of changing the Olympic operation.{{sfn|Frank|pp= 274–75}}}}

However, King was prepared to oppose proceeding with the invasion, with Nimitz's concurrence, which would have set off a major dispute within the US government:

{{blockquote|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.&nbsp;… 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.{{sfn|Frank|p= 357}}}}

===Soviet intentions===
{{Main|Proposed Soviet invasion of Hokkaido}}

[[File:Romoi-Kushiro Line.png|thumb|In a proposed invasion plan, Soviet forces were to land at the remote port of [[Rumoi, Hokkaido|Rumoi]] and occupy Hokkaido north of a line from Rumoi to [[Kushiro]]]]

Unknown to the Americans, the [[Soviet Union]] also considered invading a major Japanese island, [[Hokkaido]], by the end of August 1945,<ref name = "FP">{{cite magazine |author1-link=Sergey Radchenko |last= Radchenko|first= Sergey|date= August 5, 2015|title= Did Hiroshima Save Japan From Soviet Occupation?|url= https://foreignpolicy.com/2015/08/05/stalin_japan_hiroshima_occupation_hokkaido/|magazine= Foreign Policy|location= |publisher= |access-date= February 1, 2021}}</ref> which would have put pressure on the Allies to act sooner than November.

In the early years of World War II, the Soviets had planned on building a huge navy to catch up with the [[Western world]]. However, the [[Operation Barbarossa|German invasion of the Soviet Union]] in June 1941 forced the suspension of this plan: the Soviets had to divert most of their resources to fighting the Germans and their allies, primarily on land, throughout most of the war, leaving their navy relatively poorly equipped.<ref name = "Frank">{{cite book |last=Frank |first=Richard B. |editor-last= Hasegawa |editor-first=Tsuyoshi |title=The End of the Pacific War: Reappraisals|chapter-url= https://books.google.com/books?id=XjW49VTRhxQC&pg=PA89|publisher= Stanford University Press|date=2007 |pages=89 |chapter=Ketsu Go |isbn=978-0-80475427-9}}</ref><ref>{{cite book |title=Japanese Defence: The Search for Political Power |pages=48–60 |publisher= [[Allen & Unwin]]}}</ref>{{sfn|Allen|Polmar|1995|pp=180–185}} As a result, in [[Project Hula]] (1945), the United States transferred about 100 naval vessels out of the 180 planned to the Soviet Union in preparation for the planned Soviet entry into the war against Japan. The transferred vessels included [[amphibious assault ship]]s.

At the [[Yalta Conference]] (February 1945), the Allies had agreed that the Soviet Union would take the [[Karafuto Prefecture|southern part]] of the island of [[Sakhalin]], which Japan had [[Japanese invasion of Sakhalin|invaded]] during the 1904–1905 [[Russo-Japanese War]], and which [[Russian Empire|Russia]] had ceded in the [[Treaty of Portsmouth]] after the war (the Soviets already controlled the northern part), and the Kuril Islands, which had been assigned to Japan in the [[Treaty of Saint Petersburg (1875)|1875 Treaty of St. Petersburg]]. On the other hand, no agreement envisaged Soviet participation in the invasion of Japan itself.{{citation needed|date=July 2014}}

The Japanese had ''kamikaze'' aircraft in southern Honshu and Kyushu which would have opposed operations Olympic and Coronet. It is unknown to what extent they could have opposed Soviet landings in the far north of Japan. For comparative purposes, about 1,300 [[Allies of World War II|Western Allied]] ships deployed during the Battle of Okinawa (April–June 1945). In total, 368 ships, including 120 [[amphibious craft]], were badly damaged, and another 28, including 15 landing ships and 12 destroyers, were sunk, mostly by ''kamikazes''. The Soviets, however, had fewer than 400 ships, most of them not equipped for amphibious assault, when they declared war on Japan on August 8, 1945.<ref name="JANE">{{cite book |title=Jane's Fighting Ships of World War II |pages=180–85 |publisher= [[Random House]]}}</ref><!-- Does this include the Project Hula ships? -->

For Operation Downfall, the US military envisaged requiring more than 30 divisions for a successful invasion of the Japanese home islands. In comparison, the Soviet Union had about 11 divisions available, comparable to the 14 divisions the US estimated that it would require to invade southern Kyushu. The Soviet [[invasion of the Kuril Islands]] (August 18 – September 1, 1945) took place after Japan's capitulation on August 15. However, the Japanese forces in those islands resisted quite fiercely although some of them proved unwilling to fight after Japan's surrender on August 15. In the [[Battle of Shumshu]] (August 18–23, 1945), the Soviet Red Army had 8,821 troops that were not supported by tanks and without back-up from larger warships. The well-established Japanese garrison had 8,500 troops and fielded about 77 tanks. The battle lasted one day, with minor combat actions going on for four more after the official surrender of Japan and the garrison, during which the attacking Soviet forces lost over 516 troops and five of the 16 [[LCI(L)|landing ships]] (many of these formerly belonged to the US Navy and were later given to the Soviet Union) to Japanese [[coastal artillery]], and the Japanese lost over 256 troops. According to Soviet claims, Soviet casualties during the Battle of Shumshu totaled up to 1,567, and the Japanese suffered 1,018 casualties.<!-- Is the detailed casualty information really relevant? Also, Soviet losses typically exceeded German casualties in most battles (including Soviet victories)  -->

During World War II, the Japanese had a [[naval base]] at [[Paramushiro]] in the Kuril Islands and several bases in Hokkaido.<!-- The Imperial Japanese Navy had been mostly destroyed by then, with survivors hiding in port. --> Since Japan and the Soviet Union maintained a state of wary neutrality until the Soviet declaration of war on Japan in August 1945, Japanese observers based in Japanese-held territories in Manchuria, Korea, Sakhalin, and the Kuril Islands constantly watched the port of [[Vladivostok]] and other [[seaports]] in the Soviet Union.{{sfn|Allen|Polmar|1995|pp=115–120}}

According to [[Thomas B. Allen (author)|Thomas B. Allen]] and [[Norman Polmar]], the Soviets had carefully drawn up detailed plans for the Far East invasions, except that the landing for Hokkaido "existed in detail" only in Stalin's mind and that it was "unlikely that Stalin had interests in taking Manchuria and even taking on Hokkaido. Even if he wanted to grab as much territory in Asia as possible, he was too much focused on establishing a beachhead in Europe more so than Asia."{{sfn|Allen|Polmar|1995|pp=168–175}}