The Chōsokabe family, unlike many other daimyōs from the sengoku period (a time of constant military conflicts in Japan that lasted from about the 15th to the 17th century), traced its pedigree to at least 1180, a time before the Kamakura bakufu (shogunate) had been founded. During the Ōnin War (1467-1477), a combination of civil war and struggle for shogunal succession), the Chōsokabe family became a contender for power in Shikoku, while the attention of the Hosokawa, the hereditary shugos (overseers of a province), was focussed elsewhere.

In 1560 Chōsokabe Motochika (1538-1599) attained the family leadership (katoku), and in 1570 he began a series of military campaigns which finally gained him dominance over the entire Shikoku. Later he fought against Toyotomi Hideyoshi and was defeated by him; after that he swore fealty to Hideyoshi and received the latter's assurance that he could keep Tosa in the southern part of Shikoku as his domain.

Motochika also fought at his side in the two Korean campaigns which proved disastrous to the economic state of Tosa; after the death of Motochika, the Chōsokabe family remained loyal to Hideyoshi's heir, Toyotomi Hideyori, and sided with him against Tokugawa Ieyasu.

The last of the Chōsokabes, Motochika's fourth son Morichika, was executed along with his sons on May 11, 1615 after the defeat in the battle of Tennoji, part of the siege of the Osaka castle, so that the family became extinguished with the successful rise of the first Tokugawa shogun.

The Samurai Archives offer an excerpt from the Chōsokabe-shi Okitegaki, the Chōsokabe House Law, as laid down in 1596.

The ancient capital of the former province Tosa (today's Kochi prefecture on the island of Shikoku) was near the modern city of Nankoku. Even under the Chōsokabe, Tosa remained a relatively poor province and lacked a strong castle town.

The Chōsokabe family crest, drawn from
a copy displayed at the Samurai Archives:



Sources:
Quote from Japan: A Documentary History by David John Lu
Wikipedia
Wikimedia Commons
The Samurai Archives