Operation Downfall

Operation Downfall was the alternative to dropping the bombs.  It was the invasion of the Japanese homeland.  There were two main parts to it, Operation Olympic and Operation Coronet.  There was also a deception operation scheduled to take place before Operation Olympic, called Operation Sea-lion.

Olympic

Operation Olympic was to be the first operation in Downfall.  It consisted of a plan to capture the southern third of the southernmost main Japanese island called Kyushu.  It was to start in October 1945.  There were several airfields on Kyushu that they would use for Operation Coronet.  

One of the reasons that the potential casualty list is so large, is because of the Japanese almost bestial resistance.  Only rarely would they surrender.  In the entire battle of Okinawa only 2,000 surrendered.      

Coronet

Operation Coronet consisted of a plan to capture Kanto Plain which was very close to Tokyo.  It was on the island of Honshu.  They would use the captured airfields on Kyushu to provide air-support for the infantry on Honshu. 

One of the reasons the U.S. government was reluctant to use these operations was because of the massive expected casualty list.  They expected several million Allied casualties and tens of millions of Japanese casualties.  This plan was abandoned after the unconditional Japanese surrender.