D-Day was the first step of a massive military campaign to free Europe from Nazi control, creating a second front in Europe and trapping Germany between the Soviet Union, the United States and the United Kingdom. By forcing Hitler's armies back, the Allies reduced the pressure Germany was putting on the Soviet Union. D-Day ultimately allowed the Allies to recapture large parts of France and Germany.
D-Day had been planned before the United States joined the war. The United States encouraged the Allies to implement a "Germany first" plan of attack. On June 6, 1944, the Allies delivered hundreds of thousands of men on the beach at Normandy, catching German troops by surprise. Although Germany was aware that an attack was imminent, they did not know the exact time and location. The Allies chose Normandy as the point of entrance in part because the Germans believed that the long sea crossing made it an unlikely choice for invaders.
Codenamed Operation Overlord, D-Day owed its success to the ENIGMA machine that Germany used to send encrypted messages. The Allies had managed to break the code and were able to monitor the flow of information.
D-Day was originally planned for June 5 but a sudden storm made the invasion risky and Eisenhower postponed the invasion for a day.