World War II lasted from 1939 to 1945. It pitted the Axis powers against the Allies, which eventually included Britain, France, the Soviet Union, China, the United States, and 43 other nations. Unlike World War I, with its defensive trenches, the new global conflict was a war of aggressive movement. In the early years, things went badly for the Allies as Axis forces swept across Europe, North Africa, and Asia.
On June 22, 1941, under the code name Operation Barbarossa, Germany invaded the Soviet Union. It was the largest German military operation of World War II. German authorities planned to annihilate the Communist nation.
The Nazi invasion of Poland revealed the power of Hitler's blitzkrieg, or “lightning war.” The blitzkrieg used tank and air power technology to strike a devastating blow against the enemy.
First, the Luftwaffe, or German air force, bombed airfields, factories, towns, and cities. Screaming dive bombers attacked troops and civilians. Then fast-moving tanks and troop transports pushed their way into the defending Polish army, encircling whole divisions and forcing them to surrender.
As Germany attacked from the west, Stalin's forces invaded from the east, grabbing lands promised to them under the Nazi-Soviet Pact. Within a month, Poland ceased to exist as an independent nation. Because of Poland's location and the speed of the attacks, Britain and France could do nothing beyond declaring war on Germany.
Hitler passed the winter without much further action. Stalin's armies, however, forced the Baltic states of Estonia, Latvia, and Lithuania to host bases for the Soviet military. Soviet forces also seized part of Finland, which put up stiff but unsuccessful resistance.
In April 1940, Hitler launched a blitzkrieg against Norway and Denmark, both of which soon fell. Next, his forces slammed into the Netherlands and Belgium.