'Top Secret' maps reveal the massive Allied effort behind D-Day

As dawn broke on June 6, 1944, in northern France, the Allies began an invasion in the works for years: D-Day, the start of Operation Overlord that turned the tide against Nazi Germany.

Storming the Beaches

Following months of top secret planning, U.S. Army troops wade ashore from a landing craft to Omaha Beach, Normandy, on June 6, 1944: D-Day.
Photograph by Granger/Album

The Allied invasion of German-occupied France that began in the early hours of June 6, 1944, was long in the making. By gaining supremacy in the Atlantic in 1943, the Allies had cleared the way for a huge buildup of American troops and equipment in Great Britain. Between January and June 1944, nine million tons of supplies and 800,000 soldiers crossed the Atlantic from the United States to bolster the invasion, designated Operation Overlord.

Meanwhile, Allied pilots exploited their hard-won superiority over the diminished German Luftwaffe by blasting French railways and bridges to keep their foes from rushing reserves to Normandy when troops landed there. Anglo-American commanders battle tested in North Africa and Italy, including American Dwight D. Eisenhower and Bernard Montgomery of the United Kingdom, prepared to lead invasion troops against their old foe, German general Erwin Rommel, assigned to strengthen French coastal defenses while the bulk of the German Army struggled to hold back resurgent Soviets on the Eastern Front. (See also: Memories of D-Day come alive on the beaches where it happened.)

Planning for Operation Overlord began in London more than a year before the invasion took place. Allied staff officers led by Lt. Gen. Frederick Morgan debated where to pierce the Atlantic Wall, German coastal fortifications extending from Norway to the southwest coast of France. The shortest route to Germany lay across the Strait of Dover (Pas-de-Calais), but landing around Calais meant attacking the strongest sector of the Atlantic Wall.

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