BOOM! This was the sound that Americans heard when Japanese bombers attacked Pearl Harbor in Hawaii, a naval base, killing countless American soldiers. This led to the United States declaring war on Japan and therefore Germany, an ally of Japan. Shortly thereafter the horrible media image of fascist Germany swept across the United States, and the country's participations in the European theater of World War II heated up. Hitler had taken France, and the allied troops, along with those of the United States, had to regain control of western Europe in order to halt Hitler's movement ever farther East, perhaps even to the United States itself. After much planning, operation "Overlord" was put into effect by President Dwight D. Eisenhower in order to take back France and turn the tide of war against Hitler's German forces.
The Allies made many preparations for operation Overlord. Operation Overlord was commandeered and planned by the United States and the British. Through the winter and spring of 1944, tens of thousands of allies were sent out on convoys to meet British troops in England. Rigorous training then ensued, including training in combat for fortified forces. The American President/General Dwight D. Eisenhower was named supreme commander for the allies in Europe. The British General, Sir Frederick Morgan established an American-British headquarters known as the Chief of Staff to the Supreme Allied Commander, also known as COSSAC. COSSAC developed and designed a number of plans for the Allies; most significantly Operation Overlord, a full-scale invasion of France across the English Channel. Eisenhower felt that operation overlord sounded like a good attack plan. After reviewing the disastrous hit-and-run raid in 1942 in Dieppe, planners decided that the strength of German defenses required an intense concentration of power in a single main landing rather than small units.