Britain and France essentially acquiesced to Germany's rearmament (1935-1937), remilitarization of the Rhineland (1936), and annexation of Austria (March 1938). In September 1938, after signing away the Czech border regions, known as the Sudetenland, to Germany at the Munich conference, British and French leaders pressured France's ally, Czechoslovakia, to yield to Germany's demand for the incorporation of those regions. Despite Anglo-French guarantees of the integrity of rump Czechoslovakia, the Germans dismembered the Czechoslovak state in March 1939 in violation of the Munich agreement. Britain and France responded by guaranteeing the integrity of the Polish state. Hitler responded by negotiating a nonaggression pact with the Soviet Union in the summer of 1939. The German-Soviet Pact of August 1939, which stated that Poland was to be partitioned between the two powers, enabled Germany to attack Poland without the fear of Soviet intervention.

On September 1, 1939, Germany invaded Poland. The Polish army was defeated within weeks of the invasion. From East Prussia and Germany in the north and Silesia and Slovakia in the south, German units, with more than 2,000 tanks and over 1,000 planes, broke through Polish defenses along the border and advanced on Warsaw in a massive encirclement attack. After heavy shelling and bombing, Warsaw surrendered to the Germans on September 28, 1939. Britain and France, standing by their guarantee of Poland's border, had declared war on Germany on September 3, 1939. The Soviet Union invaded eastern Poland on September 17, 1939. The demarcation line for the partition of German- and Soviet-occupied Poland was along the Bug River.