Okinawa Prefecture was ruled by the Ryukyu Kingdom from 1429 and unofficially annexed by Japan after the Invasion of Ryukyu in 1609. Okinawa Prefecture was officially founded in 1879 by the Empire of Japan after seven years as the Ryukyu Domain, the last domain of the Han system. Okinawa Prefecture was occupied by the United States of America during the Allied occupation of Japan after World War II, and governed by the Military Government of the Ryukyu Islands from 1945 to 1950 and Civil Administration of the Ryukyu Islands from 1950 until the prefecture was returned to Japan in 1972. Okinawa Prefecture comprises just 0.6 percent of Japan's total land mass but about 26,000 (75%) of United States Forces Japan personnel are assigned to the prefecture; the continued U.S. military presence in Okinawa is controversial.