The Ryukyu Kingdom was a kingdom in the Ryukyu Islands from 1429 to 1872. It was ruled as a tributary state of the Ming dynasty by the Ryukyuan monarchy, who unified Okinawa Island to end the Sanzan period, and subsequently extended the kingdom to the Amami Islands and Sakishima Islands. The Ryukyu Kingdom played a central role in the maritime trade networks of medieval East Asia and Southeast Asia despite its small size. The Ryukyu Kingdom became a vassal state of the Satsuma Domain of Japan after the invasion of Ryukyu in 1609 but retained de jure independence until it was illegally transformed into the Ryukyu Domain by the Empire of Japan in 1872. The Ryukyu Domain was formally annexed by Japan in 1879 and reorganized as Okinawa Prefecture, and the Ryukyuan monarchy was integrated into the new Japanese nobility.