Sixty years ago yesterday, American and Allied forces landed on the beaches of Normandy, launching the greatest armed invasion in military history. The stakes could not have been higher. The success or failure of that bold assault would mean the difference between Nazi tyranny and the liberation of Europe.

By the end of June 6, 1944, 175,000 Allied troops had landed on French soil. As Gen. Omar Bradley was later to comment, the invasion force was a “thin wet line of khaki that dragged itself ashore” on D-day under fierce enemy fire. A successful invasion of Europe would mean that Germany could not win the war. Failure would have given Hitler time to use his new weapons and to divide the fragile alliance arrayed against him. Few days in history have meant so much to the future of mankind.

On one level, the D-day invasion stands as a landmark achievement in military planning. Winston S. Churchill was correct in calling the invasion plan “the most difficult and complicated operation ever to take place.” Questions of logistics, timing, and strategy called out the very best–and sometimes the worst–from the military leaders who orchestrated the war on the Allied side. Churchill and the British called for an invasion at the earliest possible moment. Joseph Stalin demanded that the Allies open a Western front to relieve pressure on beleaguered Soviet troops. President Franklin D. Roosevelt and Gen. Dwight D. Eisenhower, the supreme Allied commander, used both their powers of persuasion and considerable political acumen to delay the invasion until, in their eyes, victory was more likely than defeat.

Both sides knew that the invasion would have to come. For the last seven years, Adolf Hitler and his Nazi storm troopers had been advancing across Europe. Their Blitzkrieg strategy, matched with the power of the Wehrmacht troops, gave the Nazi regime unprecedented success–but Germany now controlled more territory than it could defend.

Adolf Hitler understood this clearly. In a directive issued November 3, 1943, Hitler instructed his General Staff that the situation in the East had reached something of a stalemate. “Not so in the West!” Hitler declared. “If the enemy here succeeds in penetrating our defense on a wide front, consequences of staggering proportions will follow within a short time.” Just weeks before the invasion, Hitler gave direct instructions to his commanders: “The destruction of the enemy’s landing attempt means more than a purely local decision on the Western Front. It is the sole decisive factor in the whole conduct of the war and hence in its final result.”

The Allies began planning “Operation Overlord” in 1942, but the most intensive period of preparation came in the three months prior to the invasion. The planning was a triumph of leadership and strategic thinking–mixed with a keen analysis of Hitler’s preconceptions. Hitler, assuming the Allies would follow the plan he would himself devise under the same circumstances, convinced himself that the Allies would land at Pas de Calais, on the French coast closest to England. Accordingly, Hitler instructed his generals to concentrate forces and fortify his “Atlantic Wall” near the port of Calais. Eisenhower and his fellow commanders finally agreed that playing into Hitler’s hand would be suicide. Instead, they aimed their sights at the beaches of Normandy, an invasion site that would stretch the Allies to the breaking point, but held the possibility of catching the Germans by surprise.

As it happened, Allied commanders did not rely on Hitler’s preconceptions alone. Gen. George S. Patton–then out of favor for a breach of military discipline–was given command of a phantom army mobilized just across the English Channel from Calais, complete with thousands of rubber tanks and assorted military paraphernalia. In a bold and costly move, Churchill agreed to seal the English coast from all civilian traffic, leaving Hitler and his commanders largely in the dark about Allied plans. The phantom army, complete with orchestrated radio traffic and the appearance of activity on the ground, did indeed blind the Nazis to the real target of the invasion–Normandy.

The timing of the operation was critical. The invasion would need to take place after the spring thaw, but before the full opportunity of summer was lost. Both sides knew that May and June were critical months of opportunity, and Stalin had been promised that an invasion would take place near May 31, but the final obstacle on the calendar was completely out of military control. As is so often the case, weather made a decisive impact on the battle, and Eisenhower made one of the most difficult decisions of his life in determining to seize a predicted break in severe storms over the English Channel to send the Allied Expeditionary Force into action on June 6.

The mobilized armies, readied to cross the channel and face the full force of Nazi might, were addressed by their Supreme Commander, known affectionately as “Ike,” with a sober message. “You are about to embark upon the Great Crusade, for which we have striven these many months. The eyes of liberty-loving people everywhere march with you. Accompanied with our brave Allies and brothers in arms on other Fronts, you will bring about the destruction of the German war machine, the elimination of Nazi tyranny over the oppressed peoples of Europe, and security for ourselves in a free world. Your task will not be an easy one. Your enemy is well-trained, well-equipped and battle-hardened. He will fight savagely.”

The massive invasion force, complete with newly-devised landing craft and ingenious floating ports, hit the beaches in the early morning of June 6, 1944. The invasion did catch the Nazis by surprise. Field Marshall Erwin Rommel, the lead German commander in the theatre, was then at home celebrating his wife’s birthday. The weather had convinced Rommel that any Allied invasion would be delayed for days. “How stupid of me,” Rommel responded when he heard the news. “How stupid of me!”

The hopes of millions rested on the 150,000 Allied troops that stormed the beaches known as Sword, Juno, Gold, Omaha, and Utah. Troops landed in amphibious craft on the beaches, parachuted onto land from C-47 transports, and rode dangerously into “controlled crashes” on Waco gliders. Many of the soldiers never made it into battle. Those who did faced a determined and fierce foe whose aim was to protect Hitler’s maniacal vision of an eternal “Third Reich.”

Casualty rates defied the imagination, as up to 90 percent of the first troops hitting the beaches were cut down by enemy fire before they could emerge from the water. The carnage along the coastline did not deter the invaders. They came in wave after wave, determined to make the invasion a success and to liberate Europe. Brig. Gen. Theodore Roosevelt, Jr. and his brigade were landed far from their planned destination. “We have landed in the wrong place,” Roosevelt told his troops. “But we will start the war from here.” For so many Allied troops, D-day represented their very first taste of battle–and they started the war right where they stood.

Once the invasion began, the real burden of the war shifted from the strategic brilliance of leaders like Eisenhower and his circle of commanders, to soldiers on the ground. As historian Stephen E. Ambrose was later to comment, “It all came down to a bunch of eighteen-to-twenty-eight-year-olds. They were magnificently trained and equipped and supported, but only a few of them had ever been in combat. Only a few had ever killed or seen a buddy killed.”

Hitler was confident his Wehrmacht troops could easily defeat the soft Americans. “It was an open decision, toward the end of Spring 1944, as to whether a democracy could produce young soldiers capable of fighting effectively against the best that Nazi Germany could produce,” Ambrose acknowledged. Hitler was wrong.

The “what if” questions of history serve both to frighten and to fascinate. This much is clear–if Operation Overlord had failed, public opinion would have turned against the Allied command, including both military and political leaders. Public support for a negotiated settlement with Hitler would have produced chaos in the Allied camp. Stalin, who had already tried making a “deal with the devil” with Hitler, was likely to negotiate a separate peace.

Hitler understood the score. “Once defeated, the enemy will never again try to invade. Quite apart from their heavy losses, they would need months to organize a fresh attempt. An invasion failure would also deliver a crushing blow to British and American morale.” Hitler was right about the cost of failure. He was devastatingly wrong about the actual outcome of the battle.

Within weeks after June 6, millions of Allied troops were pushing the Nazis back, liberating European soil and spelling the eventual doom of the Nazi regime. Paris would be liberated in August, and Germany would fall less than a year after the invasion.

Victory against Nazi Germany can be traced directly back to those brave troops who refused to be pushed back on June 6, 1944. Omar Bradley would later comment, “Every man who set foot on Omaha Beach that day was a hero.” That was true of all those brave men who faced the foe and would not flinch or falter. That “thin wet line of khaki” changed the world and reclaimed freedom for millions.

Most of the men who fought on D-day are now dead, joining the thousands of their comrades who died without ever seeing victory in the war. Thousands of others remain, now advanced in years, all bearing the scars of that awful and magnificent day. All Americans–and the world–owe these brave men our gratitude, and our pride.