On this day in 1944, Supreme Allied Commander General Dwight D. Eisenhower gives the go-ahead for largest amphibious military operation in history: Operation Overlord, code named D-Day, the Allied invasion of northern France.

By daybreak, 18,000 British and American parachutists were already on the ground. At 6:30 a.m., American troops came ashore at Utah and Omaha beaches. At Omaha, the U.S. First Division battled high seas, mist, mines, burning vehicles-and German coastal batteries, including an elite infantry division, which spewed heavy fire. Many wounded Americans ultimately drowned in the high tide. British divisions, which landed at Gold, Juno, and Sword beaches, and Canadian troops also met with heavy German fire, but by the end of the day they were able to push inland.

Despite the German resistance, Allied casualties overall were relatively light. The United States and Britain each lost about 1,000 men, and Canada 355. Before the day was over, 155,000 Allied troops would be in Normandy. However, the United States managed to get only half of the 14,000 vehicles and a quarter of the 14,500 tons of supplies they intended on shore.

Three factors were decisive in the success of the Allied invasion. First, German counterattacks were firm but sparse, enabling the Allies to create a broad bridgehead, or advanced position, from which they were able to build up enormous troop strength. Second, Allied air cover, which destroyed bridges over the Seine, forced the Germans to suffer long detours, and naval gunfire proved decisive in protecting the invasion troops. And third, division and confusion within the German ranks as to where the invasion would start and how best to defend their position helped the Allies. (Hitler, convinced another invasion was coming the next day east of the Seine River, refused to allow reserves to be pulled from that area.)

Field Marshal Bernard Montgomery, commander of Britain's Twenty-first Army Group (but under the overall command of General Eisenhower, for whom Montgomery, and his ego, proved a perennial thorn in the side), often claimed later that the invasion had come off exactly as planned. That was a boast, as evidenced by the failure to take Caen on the first day, as scheduled. While the operation was a decided success, considering the number of troops put ashore and light casualties, improvisation by courageous and quick-witted commanders also played an enormous role.