The Balkan Campaign of World War II began with the Italian invasion of Greece on 28 October 1940. In the early months of 1941, Italy's offensive had stalled and a Greek counter-offensive pushed into Albania. Germany sought, by deploying troops to Romania and Bulgaria, to aid Italy by attacking Greece from the east; while the British landed troops and aircraft to shore up Greek defences. A coup d'état in Yugoslavia on 27 March caused Hitler to order the conquest of that country.
The invasion of Yugoslavia by Germany and Italy began on 6 April, simultaneously with the new Battle of Greece. On 11 April, Hungary joined the invasion. By 17 April the Yugoslavs had signed an armistice and by 30 April all of mainland Greece was under German or Italian control. On 20 May Germany invaded Crete by air and by 1 June all remaining Greek and British forces on the island had surrendered. Although it had not participated in the attacks in April, Bulgaria occupied parts of both Yugoslavia and Greece shortly thereafter for the remainder of the war in the Balkans.
In 1919, Albania's territorial integrity was confirmed at the Paris Peace Conference after United States President Woodrow Wilson opposed a plan by the European powers to divide Albania amongst its neighbors. There were attempted backroom negotiations that ultimately failed.