(NEXSTAR) – It’s almost impossible to overstate the importance of the D-Day landings as they pertain to the outcome of World War II. If not for the Allied troops that fought on Normandy’s beaches — on June 6, 1944, and the days afterward — it’s possible the war would have dragged on well into 1946.

But what would have happened, specifically, if the Allied efforts in Normandy had failed? What if the German military secured the necessary reinforcements and fended off the invading forces with much heavier firepower?

While it’s impossible to know for certain, historians generally believe such an outcome would have changed the course of the war, and likely the state of global politics.

“If D-Day had failed — let’s say the landings at two of the five beaches had been crushed by the Germans — it would have taken many, many months — well into 1945 — before the Allies were ready to launch a second invasion,” Dr. Rob Citino, a historian and Distinguished Fellow with the National WWII Museum’s scholarly research wing in New Orleans, told Nexstar. “The D-Day invasion … had involved so many men, planes, ships — and so much advance planning — that improvising a second one might have been impossible. There simply wasn’t a second bullet in the chamber.”

It’s widely known that General Dwight D. Eisenhower, who commanded the Allied forces, was also prepared to take accountability in the event of a retreat. In a note he wrote the day before and carried in his wallet, Eisenhower jotted down a message he intended to share in the event of a failure.

“Our landings in the Cherbourg-Havre area have failed to gain a satisfactory foothold and I have withdrawn the troops,” he wrote. “My decision to attack at this time and place was based upon the best information available. The troops, the air and the Navy did all that Bravery and devotion to duty could do. If any blame or fault attaches to the attempt it is mine alone.”

It has also been speculated that Eisenhower would have submitted his resignation in the event of a retreat from Normandy, though it’s uncertain whether it would have been accepted, according to Citino.

“For Eisenhower, there was no Plan B,” Citino said.

Even failure at Omaha Beach alone would likely have derailed the Allied efforts considerably. Instead of continuing inland, the rest of the Allied forces that landed at Utah, Gold, Juno and Sword Beaches may have been forced to re-take Omaha Beach from now-fortified German forces, setting off a domino effect that almost certainly would have lengthened the war and lowered morale.

It’s also possible that Soviet Premier Joseph Stalin would have instructed his armies to advance further westward in the event a failure on the western front. If the Allies had retreated to regroup, or refocus their efforts elsewhere, the continent of Europe could look much different than it does today.

“The United States and Britain might have remained allied, but it is at least conceivable that the United States might have withdrawn to the Western Hemisphere or the Pacific as the focus of its foreign, policy, essentially abandoning Europe to Soviet rule,” historian and Distinguished Professor Thomas Schwartz, of Vanderbilt University, once remarked.

Post-war tensions with the Soviet Union — i.e., the Cold War — would have arisen much earlier, too.

That’s not to mention any ripple effects that an Allied failure may have had on the political landscape of the United States. President Franklin D. Roosevelt, having already served the better part of three terms in office, was just months away from securing a fourth. But troubling news from the front could have given rival candidate Thomas E. Dewey a boost.

“Would Dewey have defeated FDR? Probably not, but the conduct of the war would have become an issue for political debate within the U.S., and who knows where that might have ended?” Citino told Nexstar.

In the end, historians generally believe that the Germany would not have emerged victorious from WWII, or even achieved any kind of stalemate. (“Remember: The U.S. was on the verge of producing an atomic bomb,” Citino noted.) But had the Allies faltered on D-Day, it’s possible the turning point of the war could have come much later, and at a much higher cost.

“If it had lasted, let’s say, another ten months, then we could have been looking at another million additional casualties (military and civilian combined),” said Citino. Hitler, he said, would have continued to carry out his genocidal plans and the Holocaust, “as bad as it was, would have been even more horrific.”

“It was bad enough that World War II in Europe ended in 1945,” Citino said. “It would have been far worse had it ended in 1946.”