At first Germany was very successful in its military operations. In less than three months (April – June 1940), Germany conquered Denmark, Norway, the Low Countries, and France. The unexpectedly swift defeat of France resulted in an upswing in Hitler's popularity and an upsurge in war fever. Hitler made peace overtures to the new British leader Winston Churchill in July 1940, but Churchill remained dogged in his defiance. Churchill had major financial, military, and diplomatic help from President Franklin D. Roosevelt in the U.S. Hitler's bombing campaign against Britain (September 1940 – May 1941) failed. Germany's armed forces invaded the Soviet Union in June 1941 – weeks behind schedule due to the invasion of Yugoslavia – but swept forward until they reached the gates of Moscow. Hitler had assembled more than 4,000,000 troops, including 1,000,000 from his Axis allies. The Soviets had lost nearly 3,000,000 killed in action, while 3,500,000 Soviet troops were captured in the first six months of the war.

The tide began to turn in December 1941, when the invasion of the Soviet Union hit determined resistance in the Battle of Moscow and Hitler declared war on the United States in the wake of the Japanese Pearl Harbor attack. After surrender in North Africa and losing the Battle of Stalingrad in 1942–43, the Germans were forced into the defensive. By late 1944, the United States, Canada, France, and Great Britain were closing in on Germany in the West, while the Soviets were victoriously advancing in the East.

In 1944–45, Soviet forces completely or partially liberated Romania, Bulgaria, Hungary, Yugoslavia, Poland, Czechoslovakia, Austria, Denmark, and Norway. Nazi Germany collapsed as Berlin was taken by the Soviet Union's Red Army in a fight to the death on the city streets. 2,000,000 Soviet troops took part in the assault, and they faced 750,000 German troops. 78,000–305,000 Soviets were killed, while 325,000 German civilians and soldiers were killed.Hitler committed suicide on 30 April 1945. The final German Instrument of Surrender was signed on 8 May 1945.