Operation Downfall was the proposed invasion of mainland Japan by Allied Forces near the end of WWII. It was canceled because the atomic bombs were dropped and Japan surrendered, thereby removing the need for a military conquest. It would have been the largest amphibious invasion in history, and it would have been the first time that a foreign power had set foot on mainland Japanese soil (in the country's 2500 year history).
Operation Downfall had two parts: Operation Olympic (Nov 1945) and Operation Coronet (March 1946). Both were commanded by MacArthur and supported by Nimitz. Operation Olympic involved the I, V, IX, and XI Corps storming the beaches of Kyushu (the southern main island) and taking airbases to support Operation Coronet. Operation Coronet involved the First Army and the Eighth Army, as well as numerous British Commonwealth units, storming the beaches of Honshu near the capital city of Tokyo.
On the opposing side was Operation Ketsu Go, the Japanese defense of its main islands. Most of Japan's forces (air and ground) were focused on the island of Kyushu. Also, tens of millions of Japanese civilians (all able-bodied civilians, men and women) were trained in basic martial arts in order to repel the invasion. In addition, the Japanese government created numerous suicide units to repel the invasion.
All in all, it would have been one of the bloodiest battles in history. An estimated 1 million Americans and 10 million Japanese would have lost their lives.
For more information, read:
The Japanese Army Handbook by George Forty
The Pacific War Companion by Daniel MarstonMilitary History Podcast is sponsored by Armchair General Magazine