The European crisis worsened in late August 1939 with the signing of a non-aggression treaty between Germany and the Union of the Soviet Socialist Republics. The Molotov-Ribbentrop Pact (so named for the Soviet and German foreign ministers who negotiated the treaty) shocked many European leaders, who had assumed that the mutual hostility between Nazism and communism would prevent such an alliance. Germany and the Soviet Union, however, had powerful incentives to co-operate with each other. Hitler was fearful of having to fight a war on two fronts and was eager to ensure that the Soviets would not intervene in any conflict. Soviet leader Joseph Stalin was deeply distrustful of Great Britain and France, fearing that they would prefer to see the USSR take the brunt of any Nazi attack. In addition, the pact afforded the Soviets an opportunity to reclaim Russian territory lost in the 1919 peace treaties.
After the signing of the pact, Germany invaded Poland on 1 September 1939. Great Britain and France declared war on Germany. Undaunted by the British and French actions, the Wehrmacht decimated the Poles. Aided by thousands of tanks, trucks, and other machines, it swept through the plains of western Poland in a lightning quick assault that became known as a Blitzkrieg (lightning war). On 17 September, the USSR fulfilled one of the terms of the Molotov-Ribbentrop Pact by invading Poland from the east. By the end of the month, the battle for Poland was over, and the nation was partitioned between the German and Soviet invaders.
While the Soviet Union seized the Baltic States and began a military campaign against Finland, the Wehrmacht remained quiet for the rest of 1939. This period of "phony war" lasted until the spring of 1940. In April 1940, Hitler instructed his generals to resume the Blitzkrieg. The targets this time were Scandinavia and western Europe. In April, the Wehrmacht conquered Denmark and Norway. On 10 May, it attacked and quickly conquered the Netherlands and Belgium. On 10 May, Hitler also ordered his Panzer divisions -- highly mechanized and therefore mobile units of tanks and infantry -- in conjunction with the Luftwaffe, the German air force, to invade France.
The Blitzkrieg was particularly effective against France. The Wehrmacht bypassed the main French defences, such as the Maginot Line, and attacked lightly defended areas, such as the Ardennes Forest. By 20 May, the Wehrmacht was at Amiens. The next day, it reached the English Channel, near Abbeville, and hemmed in the retreating British Expeditionary Force. In late May and early June, the British evacuated their troops at Dunkirk. Almost every available vessel, including fishing boats, private yachts, and motorboats, was mobilized for the operation. On 14 June, German troops marched into Paris. Eight days later, France and Germany signed an armistice. The Wehrmacht had defeated the French army in a month, something that the German army had failed to do in four years of fighting during the First World War.
France was now divided between an area controlled by German troops and a smaller southern zone that retained nominal independence by collaborating with the Nazis. The Vichy government -- so named for the capital of the unoccupied region -- was compelled to be uniformly obedient to German directives. An underground network of French resistance movements, however, valiantly attempted to subvert Nazi rule. Opposition to the German occupation also emerged from General Charles de Gaulle and his "Free French" government in London. Nonetheless, German control of France, and of the continent, was indisputable. Europe belonged to Hitler.