As a second lieutenant, he returned to the Army's Infantry Warfare School, where he specialized in armor.

[3]: 622  After General Sosaku Suzuki, commander of the IJA 35th Army transferred to Leyte in order to better coordinate defenses against the invading Allies in the Battle of Leyte, Morozumi was left in command with the defenses of the island of Mindanao, which soon came under attack by the American 24th, 31st, and 40th Infantry Divisions and the Philippine Commonwealth military including local resistance fighters.

Morozumi was officially confirmed as commander of the IJA 35th Army after Suzuki was killed in battle on 19 April.

However, in practice, Morozumi largely ignored his appointment, knowing that communications were too poor to permit any real supervision of the other elements under his nominal command.

He surrender his forces on Mindanao to Brigadier general Joseph C. Hutchinson, Acting Commander, 31st Infantry Division on 8 September 1945.