jun 10, 1940 - Italy joins the war as an ally of Germany.

Description:

Italy declares war on Germany. ... Ever since Mussolini began to falter, Hitler had been making plans to invade Italy to keep the Allies from gaining a foothold that would situate them within easy reach of the German-occupied Balkans. On the day of Italy's surrender, Hitler launched Operation Axis, the occupation of Italy.
(HISTORY)
(URL
https://www.history.com/this-day-in-history/italy-declares-war-on-germany)