Bushi were firstly in the class which guarded the Emperor and aristocrats and suppressed the conflicts as symbolized in 'samurai,' but when the Kamakura bakufu was established through the Taira clan government by TAIRA no Kiyomori, bushi developed into the official authority which were in charge of military affairs and police activities against the Imperial Court, kokushi (provincial governors) and shoen which were traditionally ruling powers.
武士は当初、「侍」に象徴されるように天皇・貴族の警護や紛争の鎮圧を任とする階層であったが、平清盛の平氏政権を経て鎌倉幕府の成立に至り、旧来の支配権力である朝廷・国司・荘園に対して全国の軍事・警察を担う公権力に発展した。