Usually, the Union of Gayah or the Gayah Empire is referred to the same political group who shares the union named Gayah or the nation which was located in the current northwestern part of Gyongsangbuk-do and Gyongsangnam-do as the common denominator. Yet, the fact is that Gayah, who also was called Garah, was only referred to the powers in Kimhae and Goryeong.
The name, Garah, which was originated from the nation of Guyah in Kimhae, eventually included both nations as they established a federation relation with one of the six Gayah nations, Bangukuk, in Goryeong. They organized such union based on the advantage of being able to connect to the channel of Nakdong River; and also as Baekje was extending its power, it was directly affecting the power of Goryeong by threatening their right of trade.
The Union of Garah, also called Imnah Garah Union, which was an alliance between Kimhae and Goryeong, was vastly damaged since the Kimhae power was much destroyed by the invasion of the Kokuryeo army in year 400. As a result, the power of the union became unbalanced; and the union between the two countries was broken after Baekje had intervened. Meanwhile, Garah, as it continuously promoted to expand its power, its existence was later able to appear in the names of high officials of a Japanese emperor.
From 479~522, Garah seemingly claimed to be the Great Garah. This implies that it was an attempt to display their differentiation and superiority compared to the Kimhae power from which the Garah was originated. The growth of Garah was clearly shown when they were having conflicts over the supremacy in the regions of Kimun and Daesa. In addition, Baekje was calling Garah a betrayer, with contempt.
In 532, Namgarah in Kimhae became subjugated to Silla. As Namgarah, who was originally called Imnah as well, collapsed, it brought itself a consciousness of crisis. Amid such crisis situation, the name of Imnah began to be commonly used by the states at the basins of Nakdong and Nam River and this was how they hardened their bond. In the mean time, the state of Anrah seemingly dominated the initiative. And this was how the Union of Imnah was born.