On Sept. 3, 1939, the United Kingdom and France responded to the invasion of Poland by declaring war on Germany. Poland fell quickly under the German, and later, Soviet attacks. In the spring of 1940, German forces captured Denmark, Norway, the Netherlands, Belgium, and Luxembourg. The Allied forces that opposed the Germans had been unprepared for Germany's blitzkrieg (lightning war) methods. Hitler used fast-moving tanks and infantry supported by dive bombers.

In May 1940, the German army moved around France's eastern defenses and overwhelmed the French army. France fell by the end of June.

The German advance stopped at the English Channel. After a series of desperate air battles over the United Kingdom in the summer and fall of 1940, the Germans failed to gain the air superiority they needed to invade England. Hitler now turned to the east and the south. He conquered the Balkans, occupied Crete, and sent an army to northern Africa. In June 1941, a huge German force invaded the Soviet Union and drove deep into Soviet territory.

At the end of 1941, Nazi Germany dominated the continent. Hitler used his power as proof of his theory that the Germans belonged to a "master race." The Nazis ruthlessly murdered about 6 million European Jews and about 5 million Poles, Gypsies, and others. Many of these people died in the Nazi concentration camps.

Despite his army's initial success, Hitler could not defeat the Soviet Union. The Soviets continued to resist and slowly pushed the invaders back. Japan's attack on Pearl Harbor on Dec. 7, 1941, brought the United States into the war. The tide turned against Germany in 1943. The Soviets counterattacked in the east. American and British troops drove the Germans out of North Africa and invaded Italy from the south. In June 1944, the Allies invaded France. After the failure of the last German offensive in December 1944, Allied troops poured into Germany. As Soviet troops closed in on Berlin from the east, Hitler committed suicide on April 30, 1945. Germany surrendered on May 7.