It was the turning point of World War II. More than 9,000 U.S. and allied soldiers lost their lives, but the invasion of Normandy on June 6, 1944, commonly known as D-Day, allowed these 160,000 troops to gain a foothold in Nazi occupied France.    

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Today marks 73 years since the invasion. In recent years, the “D” in D-Day has been a source of speculation for many. Did it stand for “death” or “dooms” or “designated”?

You may be surprised, however, by what it might actually have meant.

The late historian Stephen E. Ambrose, who wrote a full-length book about the historic battle, explained that the “D” might simply have meant “Day.”

The Army used “H” for “Hour” and “D” for “Day,” meaning the day of an important military action. Days leading up to the military action were designated as D with the minus symbol followed by the number of days until the operation, such as D-3 or D-4. Days after the action were listed with a plus sign as in D+1 or D+5. Thus D-Day could literally have meant “Day-Day,” referring to the day the operation was to commence.

The mission, though tragic in the number of lives lost, helped turn the tides of war in the allies' favor

“It was a watershed moment not just in the history of the Second World War,” writes Tom Rogan in the Washington Examiner, “but in world history.” He continues, writing that it marked one of the world's greatest moments of courage.

With persistence, by early August, the Allies had broken out of Normandy and were racing across France. They carried with them the world's better future.

D-Day will always be the ultimate metaphor for American exceptionalism.

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