In response, the United Kingdom, France and their allies declared war against Germany, resulting in the outbreak of World War II. Germany and the Soviet Union partitioned Poland between them in late 1939 followed by the successful German offensive in Scandinavia and continental Western Europe in 1940. On 10 June 1940, Mussolini led Italy into World War II on the side of the Axis. Mussolini was aware that Italy did not have the military capacity to carry out a long war with France or Britain and waited until France was on the verge of imminent collapse before declaring war, on the assumption that the war would be short-lived. Mussolini believed that Italy could gain some territorial concessions from France and then concentrate its forces on a major offensive in Egypt. Plans by Germany to invade the United Kingdom in 1940 failed after Germany lost the aerial warfare campaign in the Battle of Britain. The war became prolonged contrary to Mussolini's plans, resulting in Italy losing battles on multiple fronts and requiring German assistance. In 1941, the Axis campaign spread to the Soviet Union after Hitler launched Operation Barbarossa. Axis forces at the height of their power controlled almost all of continental Europe, including the occupation of large portions of the Soviet Union. By 1942, Fascist Italy occupied and annexed Dalmatia from Yugoslavia, Corsica and Nice from France and controlled other territories. During World War II, the Axis Powers in Europe led by Nazi Germany participated in the extermination of millions of Jews and others in the genocide known as the Holocaust.