Editing Operation Downfall (section)

===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}}