The D-Day military invasion that helped to end World War II was one the most ambitious and consequential military campaigns in human history. In its strategy and scope—and its enormous stakes for the future of the free world—historians regard it among the greatest military achievements ever.

D-Day, code-named Operation Overlord, launched on June 6, 1944, after the commanding Allied general, Dwight D. Eisenhower, ordered the largest invasion force in history—hundreds of thousands of American, British, Canadian and other troops—to ship across across the English Channel and come ashore on the beaches of Normandy, on France’s northern coast. After almost five years of war, nearly all of Western Europe was occupied by German troops or held by fascist governments, like those of Spain and Italy. The Western Allies’ goal: to put an end to the Germany army and, by extension, to topple Adolf Hitler’s barbarous Nazi regime.

Here’s why D-Day remains an event of great magnitude, and why we owe those fighters so much:

German armies during World War II overran most of Europe and North Africa and much of the western Soviet Union. They set up murderous police states everywhere they went, then hunted down and imprisoned millions. With gas chambers and firing squads they killed 6 million Jewish people and millions more Poles, Russians, gays, disabled people and others undesirable to the Nazi regime, which sought to engineer a master Germanic race.

“It’s hard to imagine what the consequences would have been had the Allies lost,” says Timothy Rives, deputy director of the Eisenhower Presidential Library in Abilene, Kansas. “You could make the argument that they saved the world. A few months after D-Day, General Eisenhower visited a German death camp, and wrote: “We are told the American soldier does not know what he is fighting for. Now, at least, he will know what he is fighting against.”

The “D” in D-Day means simply “Day,” as in “The day we invade.” (The military had to call it something.) But to those who survived June 6, and the subsequent summer-long incursion, D-Day meant sheer terror. Raymond Hoffman, from Lowell, Massachusetts, gave an oral history interview in 1978 at the Eisenhower Library about the life-and-death fear he survived as a 22-year-old paratrooper in the U.S. Army’s 101st Airborne Division.

On D-Day he parachuted with a hard thud into a Normandy cow pasture only minutes after midnight—and he heard footsteps approaching fast, even before he could unhook himself from his parachute straps.

“Boy, here I am,” he thought. “Five minutes on the ground and I’m about to get it. And I’m flat on my back, and…I got to roll, and I can’t get to my weapon and now… I can’t find my knife! And the footsteps have stopped…and (suddenly) I am looking up into the eyes of a big, brown cow.”

That was worth a grin then. But hours later, “some mysteries in life were removed,” Hoffman said.

In a gunfight with German soldiers, where bullets flew so thick that no one dared raise their heads to look up, he removed “the mystery” he’d pondered for months—about whether fear in combat would compel him to run or to fight.

He fought. And there was no longer any mystery: “You now know what it is like to be fired upon,” he said, “as well as to fire.”

“I had some fun here one day looking up statistics, of all the stuff the Allies piled up on the beaches of southern England to support the invasion,” says Rives. “They had massive ammo dumps, and supply dumps, and in one of those supply dumps they had piled up 3,500 tons of bath soap—which Eisenhower later sent into France so the soldiers could take baths.

“He had 3 million troops under his command, and what they all devoured in just one day was stupendous,” says Rives. According to historian Rick Atkinson, commanders had “calculated daily combat consumption, from fuel to bullets to chewing gum, at 41.298 pounds per soldier. Sixty million K-rations, enough to feed the invaders for a month, were packed in 500-ton bales.”

German machine-gunners mowed down hundreds of Allied soldiers before they ever got off the landing boats onto the Normandy beaches. But Eisenhower overwhelmed them, Rives says, with 160,000 assault troops, 12,000 aircraft and 200,000 sailors manning 7,000 sea vessels.

Their losses were steep: The eight assault divisions now ashore had suffered 12,000 killed, wounded and missing, with thousands more unaccounted for, according to Atkinson. The Americans lost 8,230 of the total.

“Many were felled by 9.6-gram bullets moving at 2,000 to 4,000 feet per second,” Atkinson wrote. “Such specks of steel could destroy a world, cell by cell.”

Three thousand French civilians were killed in the invasion, mostly by Allied bombs or shell fire. By then the French had lost so much in the war that they’d run out of medical supplies. Some injured citizens were reduced to disinfecting their wounds with calvados, the local brandy fermented from apples, according to Atkinson.

But when the Allied soldiers marched inland from the beaches, the French cheered, many of them giving soldiers flowers, many of them sobbing in happiness.

No one thought victory was sure. British Prime Minister Winston Churchill had pestered Eisenhower and President Franklin Roosevelt for two years before D-Day, pleading that they avoid Normandy and instead pursue a slower, less dangerous strategy, putting more troops into Italy and southern France.

But the Germans had killed tens of millions of civilians and soldiers in the Soviet Union, and the Soviets desperately wanted the Allies to bleed the Germany army by opening up a second front of battle. Eisenhower thought it disgraceful to avoid Normandy, and thought Normandy was the best military move, not only to win but to shorten the war.

The Allies had long planned the invasion for a narrow window in the lunar cycle that would provide both maximum moonlight to illuminate landing places for gliders—and low tides at dawn to reveal the German’s extensive underwater coastal defenses. Poor weather forced Allied troops to delay the operation a day, cutting into that window. But in a stroke of luck, German forecasters predicted that gale-force winds and rough seas would deter the invasion even longer, so the Nazis redeployed some of their forces away from the coast. German Field Marshal Erwin Rommel even traveled home to celebrate his wife’s birthday, bringing her a pair of Parisian shoes.

On the night before the invasion, Eisenhower penciled himself an “In case of failure” note, to be published if necessary: “If any blame or fault attaches to the attempt, it is mine alone,” he wrote.

“Of all the documents we have from his time in the Army and in his eight years of the presidency, I regard that as our most significant document here,” Rives said of the collection at the Eisenhower Library. “It shows the character of the man who led it all.”

Eisenhower hated war. Years after the war ended, he gave a speech, with a paragraph that can be seen engraved in the marble stone wall surrounding his tomb in Abilene, Kansas.

“Every gun that is made, every warship launched, every rocket fired signifies in the final sense a theft from those who hunger and are not fed, those who are cold and are not clothed. This world in arms is not spending money alone. It is spending the sweat of its laborers, the genius of its scientists, the hopes of its children. This is not a way of life at all in any true sense.”

Most battles are quickly forgotten. But all free nations owe their culture and democracy to D-Day, which can be grouped among some of the most epic victories in history. They include George Washington’s defeat of the British army at Yorktown in 1781, which allowed the American experiment in democracy to survive, and to inspire oppressed people everywhere.

And in 490 and 480 B.C, the small armies and navies of Greece defeated the huge invading forces of the Persian Empire at the battles of Marathon and Salamis. The Greeks saved not only themselves, but their democracy, classic literature, art and architecture, philosophy and much more.

Historians put D-Day in the same category of greatness.