The Balkan conflicts
In 1941 Hitler hoped to avoid war in the Balkans. When he was informed by Mussolini that the Italians intended to invade and occupy Greece, he expressed his concern that such an action could force him to intervene. When the Italian campaigns in central Europe and North Africa faltered, the Germans occupied Greece and Yugoslavia, and were forced to engage in a struggle to suppress partisan guerrillas for the rest of the war. Italy became in all but name a vassal state of the Germans. Germany demanded that the Italians send forced laborers to supply German industry, German troops garrisoned Sicily, and the Africa Corps took over the defense of the Italian colonies in North Africa.
The performance of the Italian military and industrial base was entirely predictable, and the German High Command was aware before the invasion of France that Italy would be a drain on German manpower in the Mediterranean. German aircraft which could have been available against the British, and later the Russians, was diverted to defend the Italian possessions. Mussolini and Hitler remained personally on friendly terms, but Hitler on several occasions lamented the performance of his Mediterranean ally. The Italian military not only failed to coordinate with their German allies, the Italian navy, army, and air forces frequently failed to cooperate with each other.