Prior to the war Italy had invaded Albania and officially annexed it. Mussolini's regime declared war on Britain and France on 10 June 1940, and invaded Greece on 28 October. However, Italian forces were unable to match the Nazi successes in northwest Europe; in fact, it was not until German intervention that Greece was overrun by the Axis powers. While the Greek campaign was underway, German forces, supported by the Italians, Hungarians and the Bulgarians simultaneously invaded Yugoslavia. After the mainland was conquered, Germany invaded Crete in what is known as the Battle of Crete. With the Balkans secure, Germany and her allies attacked the Soviet Union in the largest land operation in history. The Balkans campaign delayed this invasion, and subsequent resistance movements in Albania, Yugoslavia and Greece tied up valuable Axis forces. This provided much needed and possibly decisive relief for the Soviets.