In 1940 he became commander of the Army of the Po, which he still commanded when Italy entered World War II on 10 June. The Army was kept in reserve during the brief offensive against France; during this period Vercellino drafted Plan T1, for the invasion of Ticino, which however was never put into effect. In November 1940 he was made commander of the Ninth Army, deployed in Albania, participating in the Greco-Italian War. He left command of the Army in February 1941, when he became aide-de-camp to King Victor Emmanuel III.