Over the course of Japan's Edo period, the Okinawan Kingdom of Ry?ky? sent eighteen missions to Edo, the capital of Tokugawa Japan. The unique pattern of these diplomatic exchanges evolved from models established by the Chinese, but without denoting any predetermined relationship to China or to the Chinese world order. The Kingdom became a vassal to the Japanese feudal domain of Satsuma following Satsuma's 1609 invasion of Ry?ky?, and as such were expected to pay tribute to the shogunate; the missions also served as a great source of prestige for Satsuma, the only Japanese feudal domain to claim any foreign polity, let alone a kingdom, as its vassal.