The , codenamed Operation Iceberg, was a series of battles fought in the Ryukyu Islands, centered on the island of Okinawa, and included the largest amphibious assault in the Pacific War during World War II, the 1 April 1945 invasion of the island of Okinawa itself. The 82-day-long battle lasted from 1 April until 22 June 1945. After a long campaign of island hopping, the Allies were approaching Japan, and planned to use Okinawa, a large island only away from mainland Japan, as a base for air operations on the planned invasion of the Japanese mainland (coded Operation Downfall). Four divisions of the U.S. 10th Army (the 7th, 27th, 77th, and 96th) and two Marine Divisions (the 1st and 6th) fought on the island. Their invasion was supported by naval, amphibious, and tactical air forces.