After studying in Nagasaki, Ōmura returned to his village at the age of twenty-six to practice medicine, but accepted an offer from daimyō Date Munenari of nearby Uwajima Domain in 1853 to serve as an expert in Western studies and a military school instructor in exchange for the samurai rank that he was not born into.
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Ōmura ideas for modernizing Japan's military were largely implemented after his death by his followers such as Yamagata Aritomo, Kido Takayoshi, and Yamada Akiyoshi.