Jinbō Nagamoto served as a daimyō during the Sengoku period.  Nagamoto was the head of the Jinbō clan and the deputy military governor of Etchū Province.  He also served as the first lord of Toyama Castle in Etchū.

Prior to garnering control of Etchū

Nagamoto is regarded as the son of Jinbō Norimune.  In 1520, Norimune killed himself after losing in battle against the combined forces of Hatakeyama Hisanobu, the military governor of Etchū, and Nagao Tamekage, the deputy military governor of Echigo.  As the lineal heir to Norimune, Nagamoto became the next head of the family.  Just as his father, Norimune, received one of the characters in his name from Hatakeyama Hisanori, Nagamoto received one of the characters in his name from Hatakeyama Hisanaga.  However, Norimune later acted independently of Hisanobu and died in battle, so at the time of succession by Nagamoto, the Jinbō had moved away from the Hatakeyama clan.

Nagamoto endeavored to revive the Jinbō clan which had fallen from power.  By 1531, Nagamoto had regained enough influence to deploy as a member of the allied army of the military governor for the Kyōroku-Tenbun Conflict in Kaga Province.  At this time, however, the Jinbō suffered a major loss to the ikki forces.  Around 1543, Nagamoto crossed the Jinzū River, proceeding east toward the Niikawa District.  Nagamoto ordered one of his retainers, Mizukoshi Katsushige, to build Toyama Castle.  He then triggered a major battle known as the Great Battle of Etchū that enveloped the kokujin families and Shiina Nagatsune and divided the province in half.

Nagamoto advanced further south, taking aggressive actions such as by surrounding the Saitō clan at Jōnō Castle for over a year.  In 1544, although the Great Battle of Etchū subsided through the mediation of the Noto-Hatakeyama clan, the Jinbō annexed the territory to the west of the Jōganji River while Nagamoto led the Jinbō to become the most powerful family in Etchū.

Invasion by, and surrender to, the Uesugi army

In 1559, Nagamoto began to apply pressure on the Shiina clan again, and despite mediation by Nagao Kagetora (later known as Uesugi Kenshin), he continued attacked afterwards.  As a result, in 1560, his actions invited a deployment to Etchū by Kenshin.  After a defeat by the Uesugi, Nagamoto abandoned Toyama Castle and fled to Masuyama Castle.  Nagamoto reconciled with Kenshin via mediation by the Hatakeyama clan.

Thereafter, however, he colluded with Takeda Shingen of Kai and continued pressure on the Shiina.  In the seventh month of 1562, Nagamoto was attacked against by Kenshin and defeated.  Soon after the Uesugi army returned to Echigo, he rebelled again, allying with the Etchū Ikkō-ikki based at the Shōkō and Zuisen temples and, on 9/5, at the Battle of the Jinzū River, defeated the Shiina forces who supported the Uesugi.  In the course of this significant victory for Nagamoto, those killed fighting for the Uesugi included, among others, Jinbō Minbu-taifu (a member of the same family), Jinzen Magogorō (a senior retainer of the Shiina), and Toi Nirōkurō.  Next, his forces toppled Shinjō and Horie castles, and then chased after the Shiina clan to below Matsukura Castle.  In the tenth month, Kenshin came again to support the Shiina from behind, so the overthrow of the clan was halted at the last moment.  To the contrary, the Jinbō were surrounded at their base at Masuyama Castle, and, through the mediation of the Noto-Hatakeyama clan, were compelled to surrender.

Service to, and containment of, the Uesugi clan

Nagamoto lost the territory to the east of the Jinzū River, but continued as before to govern the districts of Imizu and Nei in his territory.  There is a theory that, based on friendly relations with the Noto-Hatakeyama clan, Nagamoto contained the Uesugi clan.  However, in 1566, an internal conflict arose within the Noto-Hatakeyama clan, and after the expulsion of Hatakeyama Yoshitsuna and his son by senior retainers, Nagamoto joined with Uesugi Kenshin to support a strategy by Yoshitsuna to recover Noto Province.

In 1568, after Shiina Yasutane separated from the Uesugi and stood with an alliance between the Takeda clan of Kai and the Ikkō-ikki, the Jinbō family looked up to Nagamoto’s eldest son, Jinbō Nagazumi, while a faction opposed to the Uesugi led by Terashima Motosada came to the forefront in conflict with the faction supporting the Uesugi led by Kojima Motoshige.  Nagamoto oppressed Nagazumi’s faction, and commenced attacks against the Ikkō-ikki with whom the Jinbō formerly had close relations.  This fractured the family and led to an internecine conflict.  Owing to intervention by the Uesugi family, the faction led by Motosada was destroyed, but this also deepened the subservience of the Jinbō family to the Uesugi.  Nagazumi fled and later served Oda Nobunaga in Kyōto.

Meanwhile, Nagamoto also paid attention to developments in the capital, and maintained goodwill with Oda Nobunaga prior to his march upon Kyōto.  While backing Ashikaga Yoshiaki to become the fifteenth shōgun, Nobunaga came to the capital in the ninth month of 1569.  After the issuance of an order in the first month of 1570 to influential daimyō from across Japan to come to the capital, Nagamoto sent a representative on his behalf.  However, around this time, the Jinbō clan grew weary of the internal conflict and the authority of the family was gradually taken over by a senior retainer of Nagamoto named Kojima Motoshige who was aligned with the Uesugi.  Nagamoto undertook the rites of tonsure, adopted the name of Sōshō, and transferred the role as head of the family to his second son, Jinbō Nagashiro.  However, toward the end of 1571, he changed his position again, reconciled with the Ikkō-ikki, and stood in opposition to the Uesugi.  There are no further references to Nagamoto in historical records so it is presumed he died before long thereafter.

Decline of the Jinbō clan

The line of opposition taken by Nagamoto against the Uesugi was inherited by Nagashiro.  In 1576, after Ashikaga Yoshiaki formed a third encirclement of Oda Nobunaga, the Uesugi family made clear their opposition to the Oda and launched a large-scale invasion of the Hokuriku region.  During this campaign, Masuyama Castle was attacked and information concerning the whereabouts of Nagashiro came to an end.  At this point, the lineal branch of the Jinbō clan that Nagamoto had revived was vanquished.  The heir, Jinbō Nagazumi, served the Oda clan, and, for a while, made a come back as lord of Toyama Castle, but was later expelled by Nobunaga.  Jinbō Ujiharu from a branch of the family served the Sassa clan, followed by the Tokugawa clan.