好太王碑
In 399 AD Japan formed a military alliance with Gaya and Baekje in opposition to the growing power of Goguryeo and Silla and from then on the Japanese often dispatched military forces to protect Gaya's territorial integrity, possibly in exchange for iron or other goods. The defeat of one such Japanese expedition to Gaya is described on the Gwanggaeto Stele constructed in 414 AD. Japanese mercenaries were also allowed to serve in Korea, and the characteristically Japanese keyhole-shaped burial mounds which started to appear in Gaya and Paekche at the end of the 5th century are believed to be the tombs of those who died in Korea. While the unity of the Gaya Confederacy was falling apart under pressure from neighboring states, one of the Gayan states, Ara Gaya, even formed a joint military mission with Japan in 540 AD which was staffed by Japanese officers.