In September 1943 Keyes assumed command of the II Corps from Major General John P. Lucas and commanded it throughout the Italian Campaign, landing in Italy in mid-November and serving under Lieutenant General Mark W. Clark's United States Fifth Army. Clark, who was eight years younger than Keyes, had been a fellow student at the United States Army War College in the late 1930s. His first major battle was the Battle of San Pietro Infine and later, with Major General Fred L. Walker's 36th Infantry Division under command, his corps took part in the controversial Battle of Rapido River, part of the first Battle of Monte Cassino. The corps sustained heavy losses in the battle. The corps was then involved in Operation Diadem, the fighting on the Gothic Line and the Spring 1945 offensive in Italy, which ended the fighting in Italy.