The Mōri clan (毛利氏 Mōri-shi) was a family of daimyō, descended from Ōe no Hiromoto and established themselves in Aki Province. Their name was derived from a shōen in Mōri, Aikō District, Sagami Province. The generation of Hiromoto began to name themselves Mōri.
After the Jōkyū War, Mōri was appointed to the jitō office of a shoen in Aki Province. During the Kamakura period Mōri was one of prominent Gokenin family due to the fame of their ancestor Hiromoto. At the end of Kamakura Shogunate, Mōri was distant from the Shogunate and showed a favorable attitude to Ashikaga Takauji.
In the Sengoku period, Mōri Motonari succeeded to expand their power to the whole Aki province and then other neighboring provinces. In his generation, Mōri became the daimyo from a local Gokenin.
During the war with the Oda clan and the Ikkō-ikki clan, the Mori helped the Ikkō-ikki clan by establishing a naval trade route between each others' provincial docks and harbours, the Oda eventually nullified this by laying siege to the trade ships between the two clans and went to further disrupt trade by attempting to destroy the Mōri fleet, failing on their first encounter in 1571, the second battle took place in 1579 with the Oda sending eight O' Ataka Bune (heavily armoured ships with iron-clad plating) warships to finally destroy the Mori naval threat.
After a struggle between Toyotomi Hideyoshi, who led his army as a general of Oda Nobunaga, the two sides made peace and Mori remained as a daimyo who kept five provinces in Chūgoku. In 1600 Mōri Terumoto led the West Army in the Battle of Sekigahara nominally. The West Army lost the battle and the Mōri clan lost three eastern provinces and moved their capital from Hiroshima to currentday Hagi, Yamaguchi. The newer fief, Mōri han consisted in two provinces: Nagato province and Suo province. Derived from the former, Mōri han was referred often Chōshū han.
Taken from Wikipedia