The Ryūkyū Islands are composed of more than 140 islands, 40 of which inhabited, that lie beneath the southernmost Japanese main islands. A point of contention throughout the years, ownership of Ryūkyū was often disputed by major powers. The island of Okinawa was first ruled by warlords, called either "aji" or "anji", and was unified under the rule of Shō Hashi in the early fifteenth century (Smits 90). Eventually Okinawa conquered the rest of the Ryūkyū Islands, expanding its small kingdom. Trade was booming in East Asia in the late fifteenth and early sixteenth centuries and Ryūkyū's position as a middleman helped foster relationships with Japan and China. Once trade diminished, Ryūkyū faced the threat of invasion by Japan. In 1609, the Satsuma domain invaded the Ryūkyū Kingdom and forced the King to become their vassal in order to take advantage of its connections with China, and ruled only indirectly until the 1872 (Smits 91). Coincidentally, this actually served to promote Chinese culture. The ambiguity of Ryūkyū's political status while under Japanese control was a debate that concerned most of the elite. Even though it was under Japanese domination and its leaders were aware of this, it maintained its autonomy until 1879 (Smits 107). By this time, Ryūkyū began to come under more formal Japanese control. Emperor Meiji forced the Kingdom to become a Japanese feudal domain in 1872, and in 1879 it was made a prefecture of Japan, and then it was occupied by the United States from 1945-1972 following World War II and the Battle of Okinawa. In 1972 it was finally returned to Japan as a result of years of friction between the Okinawans and the U.S. military.