Known for having defeated the powerful Tokugawa army in the Battle of Kami river and Siege of Ueda, Masayuki is now considered one of the greatest military strategists of his era.

Also most importantly, from 1572 onwards, he joined Shingen in his campaign towards Kyoto against the Oda and Tokugawa clans and took part in the Battle of Mikatagahara (1573).

In this, Masayuki supposedly had the support of Kōsaka Masanobu, who held Kaizu Castle in Northern Shinano and was also a chief retainer of the Takeda clan.

It is said that Masayuki had intended to shelter Katsuyori and advised him to abandon Kai Province and flee towards Sanada's domain in Kōzuke(Iwabitsu Castle).

Amidst the chaos, Oda retainers who were assigned by Nobunaga to govern those territories, such as Mori Nagayoshi and Kawajiri Hidetaka amongst others, either fled or were killed by local insurrection.

[citation needed] After the chaos following the death of Nobunaga,[1] Tokugawa Ieyasu invaded Kai and Shinano province to establish control there on the consent from senior vassals of Oda clan.

[10] In response for Masayuki betrayal, a commander of Hōjō forces named Fujita Ujikuni tried to capture Numata castle.

In 1584, Tokugawa Ieyasu lead his army west towards Owari province in the Battle of Komaki and Nagakute against Hashiba Hideyoshi.

Masayuki was left in northern Shinano to keep the Uesugi clan in check and took this opportunity to subjugate small, neighboring landlords and consolidate his power in the region.

In December, as Ieyasu made peace with Hideyoshi and returned to his territory, he was pressed by Hōjō Ujinao to act on the terms of their treaty.

[12] In that treaty, among other terms, Tokugawa Ieyasu agreed to transfer Numata Castle and its adjacent lands in Kōzuke province to the Hōjō clan.

In April 1585, Ieyasu advanced his army into Kai province in a move to pressure Masayuki into abdicating Numata Castle.

Ultimately, he decided to cut relations with Tokugawa Ieyasu and once more switched allegiances by sending his second son Nobushige to Uesugi Kagekatsu as a hostage.

Months later, Tokugawa forces invaded Sanada clan's territory in northern Shinano province with 7,000 men and laid siege to Ueda Castle, which was defended by only 1,200 soldiers.

However, as Masayuki immediately ask mediation of Kagekatsu to offer his surrender to the Toyotomi clan, Hideyoshi recalled Ieyasu to abort his invasion.

At this point, Hideyoshi's political presence in Japan was too strong for the Tokugawa clan to oppose, and at his mediation, the attack was called off.

Two more years would pass until the dispute between the Sanada and Hōjō clans involving Numata Castle and adjacent areas would be mediated by Hideyoshi and resolved.

The attack was successful and the castle was seized by Hōjō forces, but by this time, Toyotomi Hideyoshi had sanctioned a rule which prohibited daimyōs from engaging in battle over private disputes.

This was the Second Battle of Ueda Castle, and, whilst it was not exactly a victory, Masayuki was able to deliver a heavy blow to Hidetada and delay his forces for long enough that they were unable to show up at the main battlefield on time.

However, the western side, led by Ishida Mitsunari, lost the main battle, and the victorious Tokugawa Ieyasu was able to redistribute fiefs at will.

Masayuki and Yukimura were initially going to be executed, but, given Nobuyuki's participation in the eastern army, they were instead exiled to Kudoyama in Kii province.