On the night of 11 November 1940, "Giulio Cesare" and the other Italian battleships were at anchor in Taranto harbor when they were attacked by 21 Fairey Swordfish torpedo bombers from the British aircraft carrier , along with several other warships. One torpedo sank "Conte di Cavour" in shallow water, but "Giulio Cesare" was not hit during the attack. She participated in the Battle of Cape Spartivento on 27 November 1940, but never got close enough to any British ships to fire at them. The ship was damaged in January 1941 by splinters from a near miss during an air raid on Naples by Vickers Wellington bombers of the Royal Air Force; repairs at Genoa were completed in early February. On 8 February, she sailed from to the Straits of Bonifacio to intercept what the Italians thought was a Malta convoy, but was actually a raid on Genoa. She failed to make contact with any British forces. She participated in the First Battle of Sirte on 17 December 1941, providing distant cover for a convoy bound for Libya, and briefly engaging the escort force of a British convoy. She also provided distant cover for another convoy to North Africa in early January 1942. "Giulio Cesare" was reduced to a training ship afterwards at Taranto and later Pola. After the Italian surrender on 9 September 1943, she steamed to Taranto, putting down a mutiny and enduring an ineffective attack by five German aircraft en route. She then sailed for Malta where she arrived on 12 September to be interned. The ship remained there until 17 June 1944 when she returned to Taranto where she remained for the next four years.