As we reflect on the 75th anniversary of the Allied landings in Normandy, many Americans still inadequately grasp why this day, of all days in World War II, was so important.

Young people often fail to see why this invasion was special or how this event shaped their own world.

In honor of those who made this history, here is why I believe we must always remember D-Day.

U.S. Gen. George Marshall was correct in 1942 when he said that only an invasion of France and campaign to Berlin would ensure the defeat of Nazi Germany, but it took fully two-and-a-half years before the Allies had the necessary amount of troops, training and war material to attempt an invasion of Northern Europe.

Adolf Hitler knew we were coming. The Germans did not know exactly where and when, but their Atlantic Wall defenses were built to blunt our forces.

The military plans for Operation Overlord were astonishingly complex: it was a massive, vast operation, full of movement, timetables and logistical schedules across the air, sea and land. Problems in any of these moving parts could jam up other areas, spread through the entire operation and lead to failure.

Even the best-laid plans were still susceptible to the fog of war. U.S. Gen. Dwight Eisenhower had to lead a fractious, multinational military force with strong personalities and disruptive opinions. Unforeseen events and uncontrollable factors, including but not limited to weather, soil conditions, spies, deception plans and pure luck, could change events and outcomes.

History was replete with failed crossings of the English Channel. The Spanish failed to invade England in 1588; Napoleon failed in 1805, and in 1940, Hitler failed to cross after Dunkirk.

All of these operations were from the continent to England, but the Allied operation to cross in the opposite direction with the largest and most complex military force ever assembled defied all odds.

Finally, the Allies had only one chance for Overlord to be successful. There was no Plan B to defeat Nazi Germany.

To understand today what was at stake, let us pretend that the Germans drove the Allies back into the English Channel that day. What would have been the results?

The immediate situation would be that it would take another two years before the Allies had the resources to attempt another invasion. Political support in the U.S. and Britain would erode. The Soviet Union would likely make a separate peace with Nazi Germany, probably forcing the U.S. and Britain to then make separate deals. The Hitler regime would survive. 

Two catastrophic results can be reliably predicted if Hitler had survived in power. First, democracy would be eliminated across Europe. Second, the Holocaust would continue until 100 percent of all people Hitler deemed to be racially inferior were murdered. As we know it, the postwar world would be unrecognizable.

But that did not happen. Instead, beginning with the bloodshed on the beaches of Normandy on June 6, 1944, and through the deaths of hundreds of thousands of Allied troops over the next 11 months on the road to Berlin, by May 1945 Nazi Germany was destroyed.

Today, we cannot forget how we inherited this world, and it is for that reason why D-Day must always be remembered—and honored.

Editor’s note: Keith Huxen, Ph.D., is senior director of research and history at the National World War II Museum in New Orleans, La.