The islands that are now the Japanese prefecture of Okinawa were once their own kingdom. The Ryukyu Kingdom was completely separate from Japan and had its own imperial dynasty. The small kingdom had no army, so it paid China to defend the kingdom. Meanwhile, the kingdom was known for its world-class traders, traveling Asia in ships bought from China. In 1609, Japan invaded and conquered the Ryukyu Kingdom and kidnapped its king. That's when things got weird.
Japan wanted to keep the fact that they ruled over the islands a secret to avoid disrupting the Ryukyu Kingdom's trade with China and other countries. The now-powerless Sho Dynasty continued to pay China for defense, while providing Japan an avenue for international trade. Read about how the Japanese got away with this deception for 270 years, and what the Sho Dynasty's descendants are doing today at Tofugu. -via the Presurfer