The Allied Invasion of Japan: A Brief Overview

The campaign to invade the Japanese Home Islands and force unconditional surrender was code-named "Operation Downfall,"
Operation Downfall was divided into two major operations, Operation Olympic, the invasion of the island of Kyushu to be executed on October 27 of 1945, and Operation Coronet, the invasion of Honshu, scheduled for the spring of 1946. As the term "D-Day" had become synonymous with the landings in France, the actual landing day was designated "X-Day". On October 1, landings were going to be made on Mainland China, to secure airfields and deceive the Japanese as to the true point of attack. This diversionary operation was codenamed "Pastel Two".

The landings were to be on two points on the Southeast shore of Kyushu, and one additional point on the Southwest shore. The objective was to clear all enemy troops from the southern part of the island and secure major ports for use unloading supplies. Eleven Army divisions and three Marine divisions were earmarked for the initial landings, with approximately 436,486 men to be landed in the first four days, three times as many as at Normandy. Follow-up forces numbered 356,902, and with air-support personnel the total number of men to be landed in the first 22 days numbered over 800,000. If these 14 divisions could not take and hold the southern part of the island, reinforcements would arrive at the rate of three divisions a month. On X+5, another division plus a brigade would capture the small islands south of Kyushu for use as radar stations, etc.
The naval forces included both Halsey's Third Fleet and Spruance's Fifth Fleet, operated together. The invasion force would include 14 fast aircraft carriers, 6 light aircraft carriers, 36 escort carriers, 20 battleships and over 1,300 troop and cargo transports.
Thousands of land-based fighters and bombers were included in the operation.

The Japanese defense plan was called Ketsu-Go. The Japanese planned to meet the Americans with 15 divisions, 7 independent mixed brigades, 3 independent tank brigades and 2 fortress units, 19 destroyers, 38 subs, 750 mini-subs, 12,750 planes (mostly kamikaze), 3300 suicide attack boats, and 4000 suicide frogmen (to attack landing craft). There were no plans to declare 'open cities' or evacuate the 2,400,000 civilians on southern Kyushu. Every male between 15 and 60 and every female between 17 and 40 (except those exempted as vital workers or for medical reasons) was expected to play a part in the defense. They were trained to fight with grenades, spears, or knives, and were supposed to use mass attacks to blur the front lines and make the Americans reluctant to use poison gas or even their overwhelming air power.

The Americans expected the Japanese to mount only a token resistance, then fall back to defend the main island of Honshu. The Japanese planned a final battle on Kyushu, defending the beaches at all costs to repel the Americans. No forces were to be held in reserve to defend Honshu; the Japanese were ready to expend their entire Army, Navy, and civilian population to defend Kyushu.

Because of this error in estimating the Japanese defense, post-war military experts estimate casualties of 94,000 killed and 234,000 wounded on the American side, and nearly 2 million killed and wounded on the Japanese side, in the first 30 days of the Kyushu operation alone.