Although the Yoritomo government was initially a private political and military group in the Kanto region, it gradually gained recognition of the official authority (the imperial court), including Emperor Goshirakawa, as the ruler of eastern Japan and established its legitimacy as a government through its war with the Taira clan (Jisho-Juei War) in the late 12th century.
ところで頼朝政権は当初、関東の私的な政治・軍事勢力に過ぎなかったが、平氏政権との内戦（治承・寿永の乱）を経るに従い、後白河天皇を中心とする公権力（朝廷）から徐々に東国支配権を認められていき、政権の正統性を獲得していった。