My uncle is a retired Infantry Office from the US Army. He spent 4 years in Korea and some time in Japan.
He was part of a think tank at the time was working on some kind of project comparing eastern and western personalities (unsure spelling). Now that you have me wracking my memory banks I will have to call him this weekend.
He said that the Army concluded that for the first wave of Toyoko the allies couldn't do it with less then 1,000,000 men.
During WWII the US Army had 110 Divisions. I don't think that could have been all at one time. Depending on the type of division that could range from 10,000 - 15,000.
So with an average of 12,500 soldier to a Division you would have needed 80 Divisions JUST FOR THE FIRST WAVE!
Thank G-d for that guy named Atom and his bomb