Early in the Edo period, with the invasion of 1609, the Ryūkyū Kingdom entered into a vassal-suzerain relationship with the Japanese Satsuma Domain, also sending a series of missions over the following two hundred and fifty years to Edo, the "de facto" capital of Tokugawa Japan. At the same time, the Kingdom continued its tributary relationship with Imperial China, both receiving and sending missions; this dual status is sometimes reflected through a four character idiom that means . Thus the political status of the Ryūkyūs vis-à-vis the rest of Japan was exceptional in at least three ways: part of the "han" system, but not directly; ruled over by kings; and the locus of semi-autonomous diplomatic ties with foreign powers, despite "sakoku" or the "closed country" policy.