Around the time of the Invasion of Ryukyu by Satsuma Domain in 1609, historian Stephen Turnbull conjectured that the Hidari Gomon was prominently displayed on flags, banners, and soldiers' uniforms. After being defeated by Satsuma, Ryukyu was allowed to retain its status as an autonomous kingdom, maintaining its traditional tributary relationship to China. Official ships bound for Satsuma thereafter displayed a Japanese-style banner featuring the Shō Dynasty's family crest while private ships were forbidden to do so. Hidenobu Itai, an expert on pre-modern Japanese ships, conjectured that by following the Japanese practice, Ryukyu was displaying its allegiance to Satsuma. The ruling Satsuma Shimazu clan in fact acquiesced in Ryukyu's dual fealty, because it remained a trade conduit between China and Japan, affording the Shimazu a loophole in the restrictions on foreign trade imposed by the Tokugawa isolationist policy. Itai further claims that because Ryukyu was ordered to conceal from China its subjugation to Satsuma, the banner is highly unlikely to have been flown during voyages to China.