The Western Front was a military theatre of World War II encompassing Denmark, Norway, Luxembourg, Belgium, the Netherlands, the United Kingdom, France, and Germany. The Italian front is considered a separate but related theatre. The Western Front's 1944–1945 phase was officially deemed the European Theater by the United States, whereas Italy fell under the Mediterranean Theater along with North Africa. The Western Front was marked by two phases of large-scale combat operations. The first phase saw the capitulation of Luxembourg, Netherlands, Belgium, and France during May and June 1940 after their defeat in the Low Countries and the northern half of France, and continued into an air war between Germany and Britain that climaxed with the Battle of Britain. The second phase consisted of large-scale ground combat, which began in June 1944 with the Allied landings in Normandy and continued until the defeat of Germany in May 1945 with its invasion.

The Allies, formally referred to as the United Nations from 1942, were an international military coalition formed during World War II (1939–1945) to oppose the Axis powers. Its principal members by the end of 1941 were the "Big Four" – United Kingdom, United States, Soviet Union, and China.