The invasion of Ryukyu by forces of the Japanese feudal domain of Satsuma took place from March to May of 1609, and marked the beginning of the Ryukyu Kingdom's status as a vassal state under the Satsuma domain. The invasion force was met with stiff resistance from the Ryukyuan military on all but one island during the campaign. Ryukyu would remain a vassal state under Satsuma, alongside its already long-established tributary relationship with China, until it was formally annexed by Japan in 1879 as the Okinawa Prefecture.