For four days in November-December 1943, as World War II raged, Franklin D. Roosevelt, Winston Churchill and Joseph Stalin met in secret in the Iranian capital of Tehran. Code named Eureka, the Tehran Conference was the first time all three Allied leaders had ever been face to face. Churchill may have exaggerated only slightly in saying that it “probably represented the greatest concentration of worldly power that had ever been seen in the history of mankind.”

Expectations for the conference ran high on all sides. Its goal was not only to agree on a strategy to crush the Axis powers of Nazi Germany and Imperial Japan, but to decide what the postwar world should look like—assuming, of course, that the Allies actually won. That was a lot to achieve in their brief time together, especially given that not one of the three men totally trusted the other two. But they all knew the stakes. Failing to get past their differences could easily prolong the war or, worse still, put Adolf Hitler and Emperor Hirohito on a path to victory.

Even arranging the conference was a test of wills. Roosevelt had tried unsuccessfully to meet with Stalin for several years, but Stalin, who was reportedly worried about assassination and afraid to fly, had always declined. When Stalin finally agreed, he insisted the meeting be held in Tehran, then under joint Russian and British control. That distant and relatively out-of-the way location made it difficult for Roosevelt who, as president, could normally be away for no more than 10 days when Congress was in session; otherwise, bills passed in his absence would become law without his signature, giving him no opportunity to exercise his veto power. Despite all that, the meeting came together.

Roosevelt, Churchill and Stalin arrived in Tehran with their own agendas. While they were united in their desire to defeat Germany and set the world on a new course, they differed sharply about how to go about it. Ironically, it was Roosevelt and Churchill—often portrayed as best buddies—who were furthest apart.

At the top of President Roosevelt’s agenda was firming up plans for Operation Overlord, a cross-channel invasion from England into northern France, better known today as D-Day. At a May 1943 conference in Washington, D.C., the U.S. and Great Britain had agreed on a tentative date of May 1, 1944 for the invasion. But by the time of the Tehran Conference there were signs that Churchill thought it should be put off, possibly into 1945.

Roosevelt’s top military advisers had long pressed for Overlord and knew that Stalin, who had been assured for several years that an invasion was imminent, was losing patience. Most ominously, according to biographer Nigel Hamilton in War and Peace: FDR's Final Odyssey (2019), “There had already been rumors of serious peace-feelers between Nazi Germany and Russia in August and September—leaving open the possibility that Stalin might simply abandon the Allies and cut a separate deal.

Roosevelt not only wanted Stalin to stay the course against Hitler, but to expand Russian operations into the Pacific and join the fight against Japan. The Soviet Union had held off declaring war on Japan, but it finally would in 1945.

Roosevelt was also thinking past the defeat of Germany and Japan. He wanted both Churchill and Stalin to sign onto his vision for a very different postwar world, with the Big Three plus China serving as “four policemen,” empowered to keep the peace. He brought with him plans for the United Nations, an organization he had named and would, according to a journalist who interviewed him shortly before his death, “consider the crowning act of his career.”

Churchill was considerably less enthusiastic about Operation Overlord, at least with the timing Roosevelt was pushing for (and that the British had agreed to the previous May). He also maintained that diverting resources from the Mediterranean theater would be premature; while Italy had officially surrendered, Rome remained in the Germans’ grasp. But the reasons for his apparent change of heart have been a matter of historical debate ever since.

In Closing the Ring, the fifth volume of his war memoirs, published in 1951, Churchill tried to defend himself against the charge that he had attempted to kill Operation Overlord, which he said had “become a legend in America.” He called the charge “nonsense” and dismissed those who disagreed with his Mediterranean strategy as “simpletons.”

While some historians have accepted Churchill’s version of events, many later ones, drawing on more recently released documents, have challenged it. Cambridge historian David Reynolds, for example, writes in his 2005 book, In Command of History, that, “Churchill suppresses or doctors key pieces of evidence” regarding his opposition to Operation Overlord in his memoirs. Hamilton maintains that Churchill did “everything possible to subvert, sabotage and postpone D-Day.”

Precisely why Churchill would have opposed D-Day at that point may never be known, but there are any number of theories. Was he concerned about excessive British casualties? Did he think the invasion would fail and give Hitler a strategic edge? Was he trying to guarantee that the broader British Empire survived the war intact? The Mediterranean region was strategically vital for that purpose, while northern France didn’t make much difference.

The ruthless Russian dictator might have gone to any length to get what he wanted at the Tehran Conference. But he ultimately didn’t have to, since his agenda and Roosevelt’s were largely aligned. That didn’t stop him from bugging Roosevelt’s rooms at the Russian embassy, where the Americans stayed. The Americans, however, figured that the rooms were bugged and made a point of playing to the hidden microphones.

What Stalin wanted most was a definite commitment that his allies would open up the long-promised Western Front in France, forcing Hitler to split his troops and fight on two fronts. He also wanted a firm date and agreed on May 1. He sided with Roosevelt against Churchill in devoting more resources to the Mediterranean, which he considered an unnecessary distraction. And, as part of the bargain, he agreed to launch an offensive on the Eastern Front to coincide with D-Day, so that Hitler couldn’t easily divert troops to repel the invasion.

Churchill made several attempts to win Stalin over to his side, but the Russian dictator was unmoved. By the end of the conference it was clear that the U.S. and the Soviets, both with far more troops than the British, would have a greater say in the course of the war from then on.

While some of Roosevelt’s critics have claimed the crafty Stalin played the president like a balalaika (a criticism also leveled after the Yalta Conference in 1945), Roosevelt came home with virtually everything he wanted, although he wouldn’t live to see all of it come to pass. Stalin did well, too, including an agreement to expand Russia’s border into former Polish territory, which he saw as a way to create a buffer between his country and Germany—and which foreshadowed his later aggression in Eastern Europe.

Churchill failed in his primary mission of putting off D-Day, but seems to have made the best of it. Importantly for the sake of Allied unity, he didn’t withdraw British troops from the planned invasion or follow through on his threat to resign as prime minister if he didn’t get his way. And when the weather-delayed invasion finally began, on June 6, 1944, he supported it with his customary eloquence.

Days after the conference ended, the three leaders released a statement that gave no hint of any dissension. “We came here with hope and determination,” the statement concluded, “We leave here, friends in fact, in spirit and in purpose.”