Germany's invasion of Poland in 1939 is generally considered to have begun World War II. But the Allies did not launch massive assaults and instead kept a defensive stance: this was called the Phoney War in Britain or Drôle de guerre — the funny sort of war — in France. It did not prevent the German army from conquering Poland in a matter of weeks with its innovative Blitzkrieg tactics, also helped by the Soviet Union's attack on Poland.

When Germany had its hands free for an attack in the west, the Battle of France began in May 1940, and the same Blitzkrieg tactics proved just as devastating there. The Wehrmacht bypassed the Maginot Line by marching through the Ardennes forest. A second German force was sent into Belgium and the Netherlands to act as a diversion to this main thrust. In six weeks of savage fighting the French lost 90,000 men.

Paris fell to the Germans on 14 June 1940, but not before the British Expeditionary Force was evacuated from Dunkirk, along with many French soldiers.

Vichy France was established on 10 July 1940 to govern the unoccupied part of France and its colonies. It was led by Philippe Pétain, the aging war hero of the First World War. Petain's representatives signed a harsh Armistice on 22 June 1940 whereby Germany kept most of the French army in camps in Germany, and France had to pay out large sums in gold and food supplies. Germany occupied three-fifths of France's territory, leaving the rest in the southeast to the new Vichy government. However, in practice, most local government was handled by the traditional French officialdom. In November 1942 all of Vichy France was finally occupied by German forces. Vichy continued in existence but it was closely supervised by the Germans.