On this day 68 years ago, nearly 3 million Allied troops readied themselves for one of the greatest military operations of world history.

D-Day. And the push that led to Hitler's defeat.

At least 160,000 of those troops landed on the shores of Normandy, France. As they stormed the beaches, General Dwight D. Eisenhower's confident words summed up the incredible significance of their mission:

"You are about to embark upon a great crusade, toward which we have striven these many months. The eyes of the world are upon you," he wrote in a famous letter sent to troops before the assault.

"We will accept nothing less than full victory! Good Luck!"

But there's another letter that he set aside "in case of failure." What if we lost?

As you may have seen before in the National Archives, General Eisenhower had doubts in the face of a "well trained, well equipped and battle-hardened" enemy. If the invasion of Normandy failed, this is the message he would have relayed to the public. How different the world would be.

Scroll down for a transcription.

Here's what it says: "Our landings in the Cherbourg-Havre area have failed to gain a satisfactory foothold and I have withdrawn the troops. My decision to attack at this time and place was based upon the best information available. The troops, the air and the Navy did all that Bravery and devotion to duty could do. If any blame or fault attaches to the attempt it is mine alone."

*He accidentally dated the letter July 5. It should have been June 5. We're sure he had a lot on his mind.