After the Ōnin War (1467–77), the shōgun largely lost control of the country beyond the immediate vicinity of the capital Kyoto, and local warlords (daimyōs) quickly sprang up to fill the resulting power vacuum, warring constantly with one another and building yamajiro ("mountain castles") to control territory.

However, to the north of Kai was the sprawling, mountainous Shinano Province, in which a number of relatively weak warlords coexisted, among them the Suwa, Ogasawara, Murakami [ja] and Takato clans.

The campaign met with mixed success, although the young Shingen (then called Harunobu) did distinguish himself by his bravery and leadership at the 1536 Battle of Un no Kuchi).

As a result Shingen was greatly outnumbered when the two sides met at Sezawa, to the southeast of Lake Suwa, but he decided to give battle anyway and took the gamble of launching a night attack through misty rain.

Over the next eleven years Shingen gradually worked his way northwards through Shinano and seemingly completed the conquest of the province in 1553 after seizing the last Murakami stronghold at Katsurao.