The Japanese were expecting an invasion for some time, and had worked to fortify the island of Kyushu (the target of Operation Olympic, to bel launched on X-Day, November 1, 1945) by moving troops from other parts of Japan. The Sixth Army estimated that on August 1, 1945, 421,000 Japanese troops were on Kyushu, comprising nine divisions and a large number of other troops. By the end of the war fourteen days later, this number had swelled to about 680,000 (14 divisions) All known landing beaches had been heavily fortified. The Japanese home islands were believed to be particularly vulnerable to gas attack due to predictable wind patterns, but Japanese troops (but not civilians) on Kyushu were suitably equipped to defend against gas. Similar to early landings in the Pacific, the Japanese aim for defending Kyushu was to attempt to crush the landing at the beachhead, making heavy use of artillery and tanks (new tank types developed by the Japanese, comparable to the U.S. M4 Sherman, had been held back in the home islands in expectation of an invasion)

On the U.S. side, the Eighth Air Force was to be redeployed to the Pacific and equipped with B-29s; this process had already begun as the war in the Pacific drew to a close. The British and Canadians were also to contribute long-range aviation in the form of "Tiger Force", 22 squadrons (later reduced to 10) of heavy bombers. The ground invasion force would have been the largest ever assembled for an amphibious operation;

U.S. Sixth Army:

Location	Divisions
Nakakoshiki Ura & Koshiki Retto (X-4)	40th Infantry Division and 158th RCT with supporting units
Kushikino (V Marine Amphibious Corps)	2nd, 3rd, and 5th Marine Divisions with supporting units
Ariake Bay (XI Corps)	Americal (23rd Infantry) Division, 1st Cavalry Division, 43rd Infantry Division, and 112th Cavalry RCT with supporting units
Miyazaki (I Corps)	25th, 33rd, and 41st Infantry Divisions with supporting units
Floating Reserve (IX Corps)	77th, 81st, and 98th Infantry Divisions with supporting units
Follow-on units (if necessary)	11th Airborne Division

Reserve:

Corps	Units
Unnamed	6th, 7th, and 96th Infantry Divisions

The invasion of Honshu, scheduled for March 1, 1946, would have been even larger; many veteran units were to be transferred from Europe.

U.S. First Army:

Corps	Units
III Marine Amphibious Corps	1st, 4th, and 6th Marine Divisions and supporting units
XXIV Corps	7th, 27th, and 96th Infantry Divisions and supporting units
Follow-on Corps (unnamed)	5th, 44th, and 86th Infantry Division and supporting units

U.S. Eighth Army:

Corps	Units
X Corps	24th, 31st, and 37th Infantry Divisions and supporting units
XIII Corps	13th and 20th Armored Divisions and supporting units
XIV Corps	6th, 32nd, and 38th Infantry Divisions and supporting units
Follow-on Corps (unnamed)	4th, 8th, and 87th Infantry Divisions and supporting units

Reserve:

Corps	Units
Follow-on units (if necessary)	97th Infantry Division, 11th Airborne Division and supporting units
Unnamed corps	2nd, 28th, and 35th Infantry Divisions and supporting units
Unnamed strategic reserve corps	91st, 95th, and 104th Infantry Divisions and supporting units

It was also planned to include a Commonwealth Corps, (X Corps) made up of up to five British, Australian, Canadian, and other Commonwealth countries' or colonies' divisions

Sources:

Allied Order of Battle for Operation Downfall

The Japanese Plans for the Defense of Kyushu (Interviews with members of the Japanese Second General Army), prepared by the office of the Assistant Chief of Staff of the U.S. Sixth Army, December 31, 1945