In 1945, the Pacific War between the Empire of Japan and the Allies of World War II had entered its fourth year. World War II was not winding down. Instead, the fighting was being prosecuted with ever-increasing fury. Of the 1.25 million battle casualties incurred by the United States in World War II, nearly one million occurred in the twelve month period from June 1944 to June 1945. December 1944 saw American battle casualties hit an all-time monthly high of 88,000 as a result of the German Ardennes Offensive. In the Pacific during this period, the Allies captured the Mariana and Palau Islands, returned to the Philippines, and invaded Borneo. The policy of bypassing Japanese forces was abandoned. In order to free troops for use elsewhere, offensives were undertaken to reduce the Japanese forces remaining in Bougainville, New Guinea and the Philippines. In April 1945, American forces had landed on Okinawa, where heavy fighting would continue until June. Along the way, the ratio of Japanese deaths to American casualties dropped from 5 to 1 in the Philippines to 2 to 1 on Okinawa. Preparations to invade Japan
Main article: Operation Downfall
Even before the surrender of Nazi Germany on 8 May 1945, plans were already underway for the largest operation of the Pacific War, Operation Downfall, the invasion of Japan. The operation had two parts: Operations Olympic and Coronet. Set to begin in October 1945, Olympic involved a series of landings by the US Sixth Army intended to capture the southern third of the southernmost main Japanese island, Kyūshū. Operation Olympic was to be followed in March 1946 by Operation Coronet, the capture of the Kantō Plain, near Tokyo on the Japanese island of Honshū by the US First, Eighth and Tenth Armies. The target date was chosen to allow for Olympic to complete its objectives, troops to be redeployed from Europe, and the Japanese winter to pass. Japan's geography made this invasion plan obvious to the Japanese as well; they were able to predict the Allied invasion plans accurately and thus adjust their defensive plan, Operation Ketsugō, accordingly. The Japanese planned an all-out defense of Kyūshū, with little left in reserve for any subsequent defense operations. Four veteran divisions were withdrawn from the Kwantung Army in Manchuria in March 1945 to strengthen the forces in Japan, and 45 new divisions were activated between February and May 1945. Most were immobile formations for coastal defence, but 16 were high quality mobile divisions. In all, there were 2.3 million Japanese Army troops prepared to defend the Japanese home islands, another 4 million Army and Navy employees, and a civilian militia of 28 million men and women. Casualty predictions varied widely, but were extremely high. The Vice Chief of the Imperial Japanese Navy General Staff, Vice Admiral Takijirō Ōnishi, predicted up to 20 million Japanese deaths.
A study from 15 June 1945 by the Joint War Plans Committee, who provided planning information to the Joint Chiefs of Staff, estimated that Olympic would result in between 130,000 and 220,000 US-casualties of which U.S. dead would be the range from 25,000 to 46,000. Delivered on 15 June 1945 after insight gained from the Battle of Okinawa, the study noted Japan's inadequate defenses due to the very effective sea blockade and the American firebombing campaign. The Chief of Staff of the United States Army,General of the Army George C. Marshall and General of the Army Douglas MacArthur signed documents agreeing with the Joint War Plans Committee estimate. The Americans were alarmed by the Japanese build up, which was accurately tracked through Ultra intelligence. United States Secretary of War Henry Lewis Stimson was sufficiently concerned about high American estimates of probable casualties to commission his own study by Quincy Wright and William Shockley. Wright and Shockley spoke with Colonels James McCormack and Dean Rusk, and examined casualty forecasts by Michael DeBakey and Gilbert Beebe. Wright and Shockley estimated the invading Allies would suffer between 1.7 and 4 million casualties in such a scenario, of whom between 400,000 and 800,000 would be dead, while Japanese casualties would have been around 5 to 10 million. Marshall began contemplating the use of a weapon which was "readily available and which assuredly can decrease the cost in American lives": poison gas. Quantities of phosgene, mustard gas, tear gas and cyanogen chloride were moved to Luzon from stockpiles in Australia and New Guinea in preparation for Operation Olympic, and General of the Army Douglas MacArthur ensured that Chemical Warfare Service units were trained in their use.