In World War II, Germany sought to defeat its opponents in a series of short campaigns in Europe. Germany quickly overran much of Europe and was victorious for more than two years. Germany defeated and occupied Poland (attacked in September 1939), Denmark (April 1940), Norway (April 1940), Belgium (May 1940), the Netherlands (May 1940), Luxembourg (May 1940), France (May 1940), Yugoslavia (April 1941), and Greece (April 1941). Yet Germany did not defeat Great Britain, which was protected from German ground attack by the English Channel and the Royal Navy. On June 22, 1941, German forces suddenly invaded the Soviet Union. But Germany proved unable to defeat the Soviet Union, which together with Great Britain and the United States turned the tide of battle and ultimately defeated Germany in May 1945.