The Battle of Okinawa, codenamed Operation Iceberg, was fought on the Ryukyu Islands of Okinawa and was the largest amphibious assault in the Pacific War of World War II. The 82-day-long battle lasted from early April until mid-June 1945. After a long campaign of island hopping, the Allies were approaching Japan, and planned to use Okinawa, a large island only 340 mi (550 km) away from mainland Japan, as a base for air operations on the planned invasion of 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 while the 2nd Marine Division remained as an amphibious reserve and was never brought ashore. The invasion was supported by naval, amphibious, and tactical air forces.