Germany attacked and conquered northwest France once WWII began, starting in May of that year. A large Allied invasion across the English Channel was being considered by 1942, after the United States had entered the war in December 1941 and the United Kingdom (which had been evacuated from the beaches of Dunkirk in May 1940 after being cut off by the Germans in the Battle of France) had begun to investigate the prospect. The Allies' preparations for an invasion across the Channel accelerated the next year. Although the Germans did not know precisely where the Allies would strike, in November 1943, Adolf Hitler (1889–1945), cognizant of the potential of an invasion along France's northern coast, put Erwin Rommel (1891–1944) in charge of leading defense efforts in the region. Hitler gave Rommel responsibility for completing the Atlantic Wall, a fortification consisting of bunkers, landmines, beaches, and water obstacles that spanned 2,400 km. General Dwight Eisenhower (1890–1969) led Operation Overlord after being named its leader in January 1944. The Germans were tricked into thinking that Pas-de-Calais (the narrowest point between Britain and France) rather than Normandy would be the primary invasion target in the months and weeks leading up to D-Day. They also misled the Germans into thinking Norway and other countries were fair game for an invasion. Fake equipment, a fictitious army led by George Patton and located in England across from Pas-de-Calais, double spies, and phony radio signals were only some of the methods utilized to pull off the hoax.

The invasion was originally scheduled to take place on June 5, 1944, the date chosen by Eisenhower. However, severe weather in the days leading up to the operation forced a 24-hour postponement. Eisenhower approved Operation Overlord on the morning of June 5 after hearing that the weather might improve on June 6. 'You are about to go upon the Great Crusade, toward which we have striven these many months,' he proclaimed to the men. All of humanity is watching you. By the evening of the same day, over 5,000 ships and landing craft had departed England for the crossing of the Channel to France, with over 11,000 aircraft providing air cover and support for the invasion.

On the morning of June 6, thousands of paratroopers and glider troops had already landed inside enemy lines to secure key infrastructure, including bridges and escape routes. The amphibious invasions started around 6:30 in the morning. In spite of minimal resistance, the British and Canadians took Gold, Juno, and Sword, and the Americans took Utah Beach. Over 2,000 American soldiers were killed or wounded in the fierce fighting at Omaha Beach. But by the end of the day, about 156,000 Allied troops had successfully assaulted the beaches of Normandy. More than 4,000 Allied soldiers were killed in the D-Day invasion, and many more were injured or went missing. More than 326,000 troops, more than 50,000 vehicles, and approximately 100,000 tons of equipment landed in Normandy on June 11. By this time, the beaches had been secured. German losses were compounded by disarray in the ranks and the absence of their legendary commander, Rommel. At first, Hitler refused to release adjacent units to assist the assault because he thought the invasion was a ploy meant to divert the Germans from an impending onslaught north of the Seine River. Delays occurred as a result of the need to call for reinforcements from further away. He was hesitant to call in armored divisions to aid in the defense as well. Effective Allied air support, which destroyed numerous strategic bridges and caused the Germans to take long detours, and effective Allied naval support, which protected advancing Allied soldiers, also slowed the Germans. For weeks after D-Day, Allied troops fought their way across the marshes and hedgerows of the Normandy countryside while encountering fierce German resistance. With over 850,000 men and 150,000 vehicles landing in Normandy by the end of June, the Allies were ready to resume their march across France.

The Battle of Normandy was officially over by the time the Allies crossed the Seine River at the end of August 1944, freed Paris, and drove the Germans out of northwest France. As the Soviet troops advanced eastward into Germany, the Allies readied themselves to enter the country. The tide of war against the Nazis began to swing with the invasion of Normandy. It was a devastating blow to Hitler's psyche, and it also stopped him from strengthening the Eastern Front against the approaching Soviets. In the spring of 1945, the Allies formally acknowledged Nazi Germany's unconditional capitulation. The suicide of Adolf Hitler occurred on April 30, one week earlier.