Until June of 1944, the war in Europe had only one front - the Eastern Front - which had already caused the deaths of millions of people. The horrific battle of Stalingrad, which remains the deadliest battle in military history, produced massive losses for the Soviet Union.
Another area of fighting - potentially a western front in France - would relieve some of the pressure and would cause Germany's forces to be spread more thinly. It would also keep Joseph Stalin from negotiating a separate peace which, in the minds of Allied leaders, was a worrisome possibility.
At the Casablanca Conference of January 1943 the Brits (who were skeptical of a successful Cross-Channel attack) agreed in principle to a 1944 invasion of the Continent. They had reason to be concerned.
Dunkerque (the photograph depicts British prisoners in 1940) and Dieppe (another bad result in 1942) had already proven in this war how disastrous a failed Cross-Channel attack could be. But Britain had great ports, and its southern shore could provide the springboard for a successfully launched invasion of German-occupied France. It was an event which Hitler expected.
Balancing the prior loss of so many young British men (who had previously died in French trenches during WWI) against the huge risk that an attempted Allied assault on France could fail, Churchill warned that the beaches of France might be "choked with the bodies of the flower of American and British manhood."
But ... what choice did the Allies have? Without an amphibious assault on French beaches, how would the Allies break Hitler's hold over Europe?
Despite the chance of failure, the Allies decided to move forward. They called their developing plan "Operation Overlord."
On the day it launched, even the Supreme Allied Commander, General Dwight Eisenhower, was uncertain the invasion would succeed. He penned a note, to be released in the event of failure, stating that all blame was entirely his.
One can see evidence of Ike's concerns in that note. He dated it "July" 5 instead of "June" 5.