As they prepared for a cross-Channel assault on France, the Western Allies built up on British soil one of the largest and most powerful invasion forces in history. For 2 months before the landing, while troops, equipment, and supplies poured into Britain, the Allied air forces bombed railroads, bridges, airfields, and fortifications in France and Belgium and continued their attacks on German industrial centers. Postponed by delays in gathering the necessary landing equipment and by weather and tidal conditions, Operation Overlord, with Eisenhower in command, began on June 6, 1944, afterward known as D-Day. Throughout the preceding night, paratroopers were dropped behind German coastal defenses to sever communications and seize key defense posts. Hundreds of warships and innumerable small craft supported the invasion. Between 6: 30 and 7: 30 a. m., waves of Allied troops moved ashore between Cherbourg and Le Havre in history's largest amphibious operation, involving approximately 5,000 ships of all kinds.

About 11,000 Allied aircraft operated over the invasion area. More than 150,000 troops disembarked at Normandy on D-Day. Because all major French ports in the north were mined and fortified, the Allies improvised two artificial harbors, with pontoons, breakwaters, and sunken ships. One of the harbors was destroyed by a severe Atlantic gale, but the other worked perfectly. Twenty pipelines below the Channel were used to bring in critical supplies of gasoline for the tanks. The Germans had anticipated an Allied invasion of western Europe at about this time but were surprised by its location.

Gen. Gerd von Rundstedt, commander of German forces in the West, had expected the Allies to take the shortest water route and land at Pas de Calais. A British intelligence operation called Ultra, having broken key German ciphers, learned of his misapprehension. To capitalize on the situation, the Allies stationed a phantom army in Kent that reinforced Rundstedt's mistaken opinion. It may also have influenced Hitler to decide against sending reserve panzer divisions to Normandy, a decision that greatly facilitated the landing and the establishment of beachheads. Yet the Germans struck back vigorously. For more than a month they resisted while Allied forces were being built up on the crowded beaches.

The defenders were under a severe handicap, however, because Hitler had been forced to send many of his troops from France to the eastern front, where the Soviets were on the offensive.