By early 1941 Hungary, Romania, and Bulgaria had joined the Axis, and German troops quickly overran Yugoslavia and Greece in April. Germany then launched massive bombing raids on Britain in preparation for a cross-Channel invasion, but, after losing the Battle of Britain, Hitler postponed the invasion indefinitely. In May German forces swept through The Netherlands and Belgium on their blitzkrieg invasion of France, forcing it to capitulate in June and establish the Vichy France regime. In April 1940 Germany overwhelmed Denmark and began its conquest of Norway. By early 1940 the Soviet Union had divided Poland with Germany, occupied the Baltic states, and subdued Finland in the Russo-Finnish War. At sea Germany conducted a damaging submarine campaign by U-boat against merchant shipping bound for Britain.
Poland's defeat was followed by a period of military inactivity on the Western Front (see Phony War). Two days later France and Britain declared war on Germany. After signing the German-Soviet Nonaggression Pact, Germany invaded Poland on Sept. Capitalizing on the reluctance of other European powers to oppose him by force, he sent troops to occupy Austria in 1938 (see Anschluss) and to annex Czechoslovakia in 1939.
He signed alliances with Italy and Japan to oppose the Soviet Union and intervened in the Spanish Civil War in the name of anticommunism. In the mid-1930s Hitler began secretly to rearm Germany, in violation of the treaty. Political and economic instability in Germany, combined with bitterness over its defeat in World War I and the harsh conditions of the Treaty of Versailles, allowed Adolf Hitler and the Nazi Party to rise to power.