Okinawa is the main island of the southern Ryukyu Archipelago. Allied strategists were attracted by its airfields within about 350 miles (560 km) of Japanese cities. In the Pacific Theater's largest amphibious operation, the first of 172,000 troops of the U.S. Tenth Army began moving ashore on April 1. Within three weeks, they held four-fifths of the island, but organized Japanese resistance continued until June 17.