On his return to Japan, Sakai was again assigned to staff positions, but due to his fluency in French and European experience, was selected to participate in the Japanese delegation to the Versailles Peace Treaty negotiations.

On his return to Japan, he was given command of the IJA 22nd Infantry Regiment from 1929 to 1931 and served as an instructor at the Army War College from 1931 to 1934.

In 1936, Sakai was appointed commander of the IJA 24th Infantry Brigade, and was assigned to China with tensions rising in the prelude to the Second Sino-Japanese War.

During the final stages of the Pacific War, he was recalled to active duty, but served in an advisory capacity to the Army General Staff from November 1943.

Although not a member of the Imperial Way Faction, he had close ties to Genera Heisuke Yanagawa and provided information to Fumimaro Konoe and others who were attempting to remove Prime Minister Hideki Tojo from power.