19391941

1939–1941 covers the early years of World War II, a period in which the conflict broadened from the invasion of Poland to major campaigns across Europe and the beginnings of global warfare. The period saw the emergence of the Axis and Allied blocs, major diplomatic shifts, and the mobilization of economies and societies for total war.

In 1939 the war began with Germany’s invasion of Poland on September 1, followed by the Soviet Union invading eastern Poland on September 17. Britain and France declared war on Germany on September 3. The year also saw the Molotov–Ribbentrop Pact, a nonaggression treaty between Germany and the Soviet Union, which included secret protocols dividing Eastern Europe. The Winter War between the Soviet Union and Finland occurred late 1939 into early 1940.

In 1940, Axis expansion accelerated. Germany and its allies occupied Denmark and Norway in April, then overran Belgium, the Netherlands, and Luxembourg in May–June. France surrendered in June, leading to the establishment of the Vichy regime. The Battle of Britain, fought in the air over the United Kingdom in 1940, marked the first major campaign fought entirely by air forces. Italy entered the war on the side of the Axis in 1940.

In 1941 the conflict expanded further. The United States enacted Lend-Lease aid to Allies in March, signaling growing American involvement. Germany launched Operation Barbarossa, the invasion of the Soviet Union, in June. In December, Japan’s attack on Pearl Harbor brought the United States into the war, and Germany and Italy declared war on the United States, broadening the war to the Pacific and North Africa as well as Europe.

The period ended with the United States formally in the war and the conflict taking on a truly global scope.