There were 36,525 days in the twentieth century. Of these, none was more consequential than June 6th, 1944. D-Day: the Allied invasion of Normandy in Nazi-occupied France. It did not end World War II, but without it, the Nazi war machine would not and could not have been defeated.

We, of course, know the good guys – America, England and its allies – won. But in 1944, there was no certainty of success. In fact, there was just as much doubt as confidence. Winston Churchill’s senior advisor, Field Marshal Brooke, wrote in his diary, “I am very uneasy about the whole operation. It may well be the most ghastly disaster of the whole war.”

Brooke’s fears were entirely reasonable.

First there were tens of thousands of men and millions of tons of material and supplies that had to be moved one hundred miles across one of the roughest bodies of water in the world – the English Channel. And it had to be kept secret. If the Germans knew where and when the allies were landing, they could mass forces against them and turn the beaches of northern France into killing fields.

To prevent this, the Allies took every possible precaution. Their air forces destroyed bridges, roads and railways that might be used by the Germans to rush troops to the invasion site. Everyone knew the attack was coming; the key was to keep the Germans guessing.

Fake radio chatter was broadcast to suggest the beaches near Calais would be the landing point. Double agents leaked fake details of units forming in South East England. And movie set designers built phony tanks, planes and ships to support the ruse of an army preparing to cross near Dover for the benefit of German reconnaissance pilots and spies.

The Germans swallowed it all. But the Nazis were not the only enemy the Allied forces faced. Mother Nature was just as threatening.

The 23,000 paratroopers and glider-borne infantry jumping into Normandy needed moderate winds to be effective. The twelve thousand Allied aircraft needed clear skies. The invasion fleet of six thousand vessels needed calm seas. And there had to be a low tide to expose Nazi obstacles and mines. When high winds and rain began pummeling the Channel, Allied supreme commander General Dwight Eisenhower postponed the invasion date of June 5th by twenty-four hours. That might not sound like a significant delay, but it was. All forces were concentrated and ready to go. All the plans, all the deceptions, could be exposed at any moment. Then came a new forecast. The weather appeared to be breaking. There might be a 12-hour window of opportunity.

Eisenhower gave the order: We go. Immediately, the greatest invasion fleet ever assembled set sail. On board were over 130,000 young soldiers.

Consider for a moment who these soldiers were. The average age of the American GI was 21. Most had never seen combat or even been fifty miles from their hometown. As they sailed toward the French shoreline, Eisenhower wrote a press release in case of catastrophe. D-Day was an all-or-nothing affair. A new invasion strategy would take months, if not years, to devise.

The initial battle reports were seriously troubling. At Omaha Beach, overlooked by cliffs honeycombed with trenches, cannon and machine-guns, the Americans took heavy losses. “I might have killed hundreds that morning,” reflected German soldier Hein Severloh, manning one of the bunkers. The rough surf also took its toll. Dr. Harold Baumgarten, with the U.S Army’s 116th Infantry, remembered, “Some of the fellows were pulled under by their wet combat jackets and heavy equipment. We couldn’t help; they just drowned.” Further along, Army Rangers also took heavy casualties as they scaled the cliffs under intense gunfire. However, by mid-day – with US naval support – the Germans, low on supplies and ammunition, began to fold. Nazi reinforcements, including hundreds of tanks, which might have made all the difference, were not ordered to Normandy until the afternoon. Before the Germans could mount an effective counter-attack, the Allies had secured all five landing beaches.

Churchill had expected twenty thousand to be killed on D-Day. Fortunately, heavy though they were, the losses were much lower. Of the 156,000 Allied personnel who hit the beaches that day, ten thousand became casualties. Of these, five thousand were killed.

No one died in vain.

Their sacrifice meant an end to Nazi Germany and the Holocaust. Another year of bitter fighting lay ahead, but D-Day – June 6, 1944, was a pivotal step on the road to forever removing the Nazi tyranny from Europe and the world.

I’m Peter Caddick-Adams, author of Sand and Steel: A New History of D-Day, for Prager University.