During the last phase of World War II Ambrosio supported the fall of Benito Mussolini and Italy's eventual renunciation of the German alliance.

[1] Ambrosio's early actions in World War II included leading the Italian offensive during the Invasion of Yugoslavia in 1941.

After brief but heavy fighting, the 2nd Army under his command attacked from the north reaching Ljubljana and the outskirts of Zadar by April 11.

In May 1943, after a devastating loss at Tunis and the Allied invasion of Sicily, Ambrosio attempted to convince Mussolini to pull Italy out of the war and end the alliance with Germany.

After Mussolini's fall in July, Ambrosio continued as Chief of Staff in Pietro Badoglio's military government.