Seventy-two years ago Monday we, in the words of Supreme Allied commander Dwight Eisenhower, “embarked on a great crusade.”

On June 6, 1944, more than 160,000 Allied troops landed on a heavily reinforced coastline to fight Nazi Germany on the beaches of France. Some units walked ashore free from enemy fire, while others, especially those on Omaha Beach, did anything but. The cost in Allied lives was high that day, with more than 9,000 troops killed or wounded.

It was the so-called Day of Days, and by far is the most famous day in the Western canon of World War II literature and film. Its importance cannot be overstated.

And because of weather, it almost didn’t happen June 6.

Incredibly, even with all the technological advances brought about by war, in 1944 meteorology was very much in its infancy. There was no first-warning weather, no 10-day forecasts. There wasn’t even CBS-6’s Nikki-Dee Ray to brighten your morning. Doppler radar was invented during World War II, but it was being used to support U.S. Navy air combat activities, and not to tell us if we should wear rain boots.

To put it in perspective, years of planning and billions of dollars had gone into this one day, yet a measly storm was threatening to ruin it all.

Eisenhower and his team had chosen June 5, 6 and 7 as the three-day window in which the attack could happen. This was because they needed a full moon for the nighttime glider operations and the ensuing low tide the next morning, so that the Germans’ underwater defenses would be more visible to the landing craft operators. Optimal, aka calm, weather conditions also were preferred because rough seas could capsize landing craft and overcast skies would limit the all-important air support. Unfortunately, the English Channel rarely provides all of these things at once.

The decision of when to go ultimately was Eisenhower’s, but his main source for weather predictions was Group Capt. James Stagg, who headed a team of weather experts from the Royal Navy, British Meteorological Office and the U.S. Army Air Forces.

Had Ike given the go-ahead, and a storm thrown the invasion into chaos before it even hit the beach, the Germans would have been given the invaluable information that the attack was coming in Normandy and not the Pas-de-Calais as many of their brass believed it would.

Then they would have had the time to focus their powerful tank reserves. This almost surely would have doomed the Allied attack. It wouldn’t have lost the war, but it would have set it back years.

Needless to say, Eisenhower had a lot on his plate.

The decision to delay the operation for 24 hours was made, and the thousands of men, many of whom already were at the airfield in planes and on ships ready to roll, were made to stand down.

Stagg then informed Eisenhower that he and his team were predicting a short window, a lull in the storm, on June 6. If they didn’t go on this date, the next available attack window was two long weeks away.

Fortunately, the German meteorological team didn’t see this window, and they predicted that there couldn’t and wouldn’t be an attack in the next few days. They were putting a rainout on the books, so to speak.

The decision to risk it all on a window of opportunity wore heavily on Eisenhower. All of the planning would come down to what essentially was a roll of the dice. After consulting his top subordinates, Ike famously looked down for a few seconds, and then looking up. His knotted brow suddenly cleared, and he said something along the lines of, “Let’s go.”

What he said exactly is in dispute, but there was no doubt about his intent.

“The inescapable consequences of postponement,” Ike wrote in his 1948 memoir, “Crusade in Europe,” “were almost too bitter to contemplate.”

The weather on the morning of the 6th wasn’t ideal, but as Stagg and his team predicted, it cleared enough by noon and the attack, although not perfect, succeeded. The second front was opened.

Eleven months later Germany capitulated.

If not because of a fateful break in the weather, it could easily have been much, much longer. A defeat on D-Day would have allowed the Germans to shift more resources to the Eastern Front to slow, although not stop, the Russian advance — and then, who knows how long the war would have lasted?

What we do know is that many more men and women would have died.

Remember that the next time the clouds part and the sun starts to shine through. S

Jack Lauterback also is co-host of “Mornings with Melissa and Jack” on 103.7 Play weekdays from 6-9. Connect with him at letters@styleweekly.com, or on Twitter at jackgoesforth.