World War II began in September 1939 with the German invasion of Poland but, despite a declaration of war against Germany by Britain and France, neither country launched any significant land-based military operations against the German occupation. This period was known as the Phoney War due to Allied inaction despite existing defensive alliances with Poland. The Phoney War ended with the German invasion of the Low Countries in May 1940, after which the northern half of France itself was quickly overrun. With the country in crisis and many of the top figures in government having resigned, the new French Prime Minister Philippe Pétain signed an armistice with Germany on 22 June in the Compiègne Forest, allowing German occupation of northern France. The new French Vichy government was allowed to administer the southern zone libre. Meanwhile, the Soviet Union had annexed a number of eastern European states and Italy began a colonial expansion into North Africa.