Sakuma was born in Abu District, Nagato Province (present day Hagi, Yamaguchi), as the younger son of Okamura Magoshichi, a "samurai" of Chōshū Domain, and was later adopted into the Sakuma family. He studied Western military science under Ōmura Masujirō and was a company commander defending the domain against the Second Chōshū expedition mounted by the Tokugawa shogunate in 1866. He subsequently served in the Boshin War of the Meiji restoration with distinction at the Battle of Aizu. In 1872, he entered the fledgling Imperial Japanese Army as a captain.