On graduation at the head of his class in December 1903, he was promoted to captain, and made a squad leader in the IJA 36th Infantry Regiment, which he accompanied to Manchuria during the Russo-Japanese War of 1904-1905.

In August 1931, Watanabe was recalled to Tokyo and promoted to full general and military councilor with oversight over Army aviation.

In July 1935, he was promoted to one of the most prestigious posts within the Imperial Japanese Army, that of Inspector General of Military Training, replacing Jinzaburō Masaki.

Masaki was a close associate of General Sadao Araki and his Imperial Way Faction, promoting totalitarian, militarist, and expansionist ideals, whereas Watanabe had a reputation as a moderate.

Rebel soldiers surprised Watanabe at home in Ogikubo during the February 26 Incident, machine-gunning him to death in front of his wife and daughter.

Watanabe was posthumously awarded the Order of the Rising Sun with Paulownia Flowers and given the honorific title of Junior Second Court Rank.