Italy's entry into World War II on 10 June 1940 opened a vast new theater of operations--the Mediterranean area. During its first year the war had been fought in central and northern Europe, the center of gravity moving successively from Poland to Finland to Norway to France. Italy's participation, plus the fact that the British Isles did not succumb to the German aerial bombardment after the fall of France, turned the attention of the belligerents to the Mediterranean Sea, where Great Britain became involved in a grim struggle to maintain her supremacy. The Balkans and North Africa were soon focal points of military activity in the Mediterranean theater, and operations in those two areas furnish excellent studies of modern war in two types of terrain, mountain and desert. The two campaigns proceeded simultaneously for a while and had a profound influence on one another. Consequently the necessity of describing them separately must not hide the fact that each was a part of the great struggle for control of the Mediterranean and its surrounding shores. The Balkan campaign, which will be discussed in these pages, may be divided into three phases: the Italo-Greek War, the invasion of Yugoslavia and Greece, and the conquest of Crete. This account of the campaign in the Balkans has been written for use in the instruction of cadets at the United States Military-Academy. It is based for the most part on material prepared by the Military Intelligence Service, War Department.