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.

[1] In Japan, the war was called the Ryukyu Expansion (琉球征伐, Ryūkyū Seibatsu) or the Entry into Ryukyu (琉球入り, Ryūkyū iri) during the Edo period,[2] and was called the Okinawa Expansion Campaign (征縄の役, Sei Nawa no Eki) by many Japanese scholars before WWII.

The two regions had been engaged in trade for at least several centuries and possibly for far longer than that; in addition, Ryukyu at times had paid tribute to the Ashikaga shogunate (1336–1573) of Japan as it did to China since 1372.

In the final decades of the 16th century, the Shimazu clan, along with Toyotomi Hideyoshi, who ruled Japan from 1582 to 1598, requested or demanded various types of aid or service from the kingdom on a number of occasions.

Shō Nei sent a tribute ship, the Aya-Bune, to Satsuma in February or March of 1592,[4][5] and agreed to provide approximately half of his allocated burden in preparation for the invasion of Korea in 1593.

Following Hideyoshi's death in 1598, and Tokugawa Ieyasu's subsequent rise to power, Shō Nei was asked by Satsuma to formally submit to the new shogunate, a request which was ignored.

"[23] Kikuin and his diplomatic mission (Kian was an assistant) left the Ryukyuan royal capital, Shuri at 8am, April 26, and arrived at Kuraha[24] at 12pm.

At 2 PM, May 1, the Satsuma ships entered Naha harbor, and immediately held peace talks at Oyamise (親見世).

On December 24, he arrived at Kagoshima, where he was forced to formally surrender and to declare a number of oaths to the Shimazu clan.

In the king's absence, Kabayama Hisataka and his deputy Honda Chikamasa governed the islands on behalf of their lord Shimazu Tadatsune.

14 samurai officials from Satsuma, along with 163 of their staff,[31] examined the kingdom's political structures and economic productivity, and conducted land surveys of all the islands.

The oaths also included stipulations that the kingdom admit its culpability in ignoring and rejecting numerous requests for materials and for manpower, that the invasion was justified and deserved, and that the lord of Satsuma was merciful and kind in allowing the king and his officers to return home and to remain in power.

Tei Dō, a royal councilor and commander of the kingdom's defense against the invasion, refused to sign the oaths, and was subsequently beheaded.

This framework of guidelines was largely set down by a document sometimes called the Fifteen Injunctions (掟十五ヶ条, Okite jūgo-ka-jō), which accompanied the oaths signed in Kagoshima in 1611, and which detailed political and economic restrictions placed upon the Kingdom.

Prohibitions on foreign trade, diplomacy, and travel outside of that officially permitted by Satsuma were among the chief elements of these injunctions.

Ryukyu's extensive trade relations with China, Korea, and Southeast Asia were turned to Satsuma's interests, and various laws were put into place forbidding interactions between Japanese and Ryukyuans, and travel between the two island nations.