Winston Churchill, Franklin Roosevelt, and Joseph Stalin were an odd trio. Churchill, the United Kingdom’s prime minister, was a bullish aristocrat famous for his brandy and cigars while Roosevelt, the U.S. president, had a well-known antipathy to the British Empire. Stalin’s differences with the two were stark: The Soviet dictator was responsible for the murder of millions of his own citizens. Yet when Adolf Hitler invaded the Soviet Union in 1941, these three, larger-than-life leaders joined forces to win World War II, as Winston Groom explains in his new book, The Allies, which is published by National Geographic. (Learn about a daring mission to stop a Nazi atomic bomb.)

Speaking from his home in Point Clear, Alabama, Groom describes how “fake news” about the Soviet Union blinded Roosevelt to Stalin’s character and intentions, how Churchill became a powerful symbol of courage and resistance for the British, and what lessons these three giants of history hold for us today.

You start your book at the conference in Tehran in November 1943, which Churchill called “the greatest concentration of political power the world had yet seen.” Put us inside that moment—and describe the three protagonists, and the differences between them.

Churchill and Roosevelt had been trying to meet with Stalin since the Americans got into the war. Stalin put them off constantly, saying he was too busy and had to be at the Russian front. In fact, he never got within 100 miles of the front. [Laughs] But Stalin didn’t like to fly. So the Allies arranged to meet in Tehran, which Stalin could reach with his personal train.

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These three men were allies but unlikely allies. None of them fully trusted the others. Roosevelt didn’t trust Churchill because he didn’t like empires and Great Britain was the greatest empire the world had ever seen. Churchill didn’t fully trust Roosevelt because he knew that he had a political situation at home, where many people were opposed to American involvement in the war. And Stalin didn’t trust anyone, including himself, so he said. They were all a little suspicious, but even more so when Churchill and Roosevelt arrived in Tehran and were informed that there was a kind of assassination plot under foot. So, they were forced to stay in the Russian Embassy where they would be spied upon.

One of the crucial differences between Churchill and Roosevelt was their attitude to Stalin, whom Roosevelt affectionately called “Uncle Joe.” Was Roosevelt blind to Stalin’s brutality—or was this just realpolitik?

I don’t know! I don’t think anybody really knows. Churchill called him Uncle Joe, too, between themselves. But Roosevelt was a little blind because he got most of his information from the New York Times, as many Americans did in those days, including the U.S. State Department. The Times’ correspondent in Moscow, Walter Duranty, was a real low-life. He was an alcoholic womanizer, and the Soviets supplied him with a fine apartment and women and booze. In exchange, he painted a rosy picture of the Soviet Union, despite the fact that millions of people were being murdered and starved to death. Almost the first thing Roosevelt did, when he got into office, was to recognize the Soviet government.

Of course, after we got into the war, we didn’t want to cast aspersions on one of our two great allies. So they tried to paint a rosy picture of the Soviet Union from the White House and Department of State. Churchill didn’t buy into it. But he recognized that you have to have such allies and there was no choice. Once the Germans attacked the Soviet Union, Stalin became an ally whether we liked it or not. And he was a great ally to have, too. Had the Soviets not joined the Allies, heaven knows what would have happened.

Animation: How Three Men You Probably Never Heard of Helped End WWII This short animation by Mike Brookes is a tribute to those heroic men of H.M.S. Petard who ultimately helped to win WWII.

I was surprised to learn that Stalin cut his teeth as a leader as a bank robber. Tell us about this—and how his early life shaped his political career.

[Laughs] He actually started out to become a priest! His mother sent him to Greek Orthodox divinity church until he was 16, but he was expelled for reading improper literature, which was probably some kind of Communist propaganda. After that, he became a Marxist-Leninist. He made a career raising money for the Marxists the only way he knew how, which was to steal. They were into bank robbing and extortion and all sorts of activities like that. He did that until the Russian Revolution, 10 years later, at which point he became a party member and had a more legitimate job. He was a very brutal man, who followed the doctrine that the end justifies the means. He was quoted as saying something to the effect that, the death of one man was a tragedy; the death of a million men is a statistic.

View Images The Allies landed roughly 156,000 troops in Normandy on June 6, 1944—D-Day. Photograph by IWM, Camera Press/Redux

Churchill’s famous V-sign became a symbol of Britain’s resistance to Hitler. How important was he to the survival of his country in the Second World War?

He was enormous. I don’t know what would have happened if it had not been for Churchill. In the recent movie Darkest Hour, which I have not seen, there’s an episode in which Lord Halifax, the foreign minister, asks whether it’s time to see what sort of terms the Nazis will offer. In the movie, Churchill dithers about it, then goes down into the Tube to ask ordinary British citizens what they thought about it. That’s all hogwash! [Laughs] What Churchill actually did was say, “If this long island story of ours is to end at last, let it end only when each one of us lies choking in his own blood upon the ground.” That’s what he told them.

Churchill was everywhere! The talks that he gave over the radio were terrifically inspiring. He knew how to marshal people without being gauche and was very, very good at what he did. He never wavered. Churchill was a soldier. He went to Sandhurst, the British version of West Point, and fought for years in Afghanistan as well as in Africa and India. Then he fought as a colonel in World War I in the trenches in France, so he knew what he was talking about. He was a good soldier and a leader.

I was shocked to learn, as some of our readers may be, that the U.S. provided Stalin with vast amounts of weapons, which were sometimes then used by the dictator against his own people. Why did Roosevelt make this decision?

He really had no choice. Stalin had his back to the wall from the time the Germans attacked him until late in 1943. The Soviets needed every weapon they could get: tanks, artillery, even radios. If the Germans had succeeded in stifling the Soviet Union, Operation Overlord [the code name for D-Day] would have been problematic because the Germans could have marshalled two or three times the number of soldiers that they had when we actually made the invasion.

I think the Russians probably only used a small proportion of the weapons we gave them on their own people. And when you are in a fight like that, you’ve got to use everything you can. What Roosevelt got in return was Soviet resistance to the Germans. And that was enough for him.

Churchill, another recent movie, was criticized for its portrayal of the British leader’s ambivalence about the D-Day landings. Talk us through Operation Overlord.

I don’t think ambivalence is the right word to use. The Americans wanted to land immediately in 1942 but Churchill, being a soldier and having experienced warfare first hand particularly at Gallipoli, knew that a seaborne landing is the most dangerous military operation in the book. And he knew that if we didn’t do it right the first time there wasn’t going to be a second chance. England had lost a sizeable amount of armaments and men at Dunkirk, and if they lost another army they would have been completely vulnerable.

Churchill sought to take the war elsewhere, to North Africa or Italy. He wanted to go for what he called the soft underbelly of Europe, via the Mediterranean and then up through the old Austrian empire. He also sought to postpone the invasion. He knew the Americans were going to insist on it and that Great Britain would play a key role but he finally got it put off until 1944.

It was the largest seaborne invasion the world had ever seen and it probably will stay that way. We put about a million men on the beaches within the first couple of days, and millions more after that. It took over a month and a half to break out of Normandy. But we had fooled the Germans into thinking that the invasion would be at the Pas de Calais, 150 miles to the north, and it worked. D-Day marked the beginning of the end for Nazi Germany.

View Images A young man looks up at the memorial to the Allied forces who landed on Omaha Beanch in Normandy in 1944. Photograph by Gruffydd Thomas, Alamy

You describe how Churchill, at the Yalta Conference in 1945, scribbled down on a sheet of paper the division of the post-war world, which resulted in Stalin annexing much of Eastern Europe. Roosevelt even asked Stalin not to occupy Poland until after the U.S. elections. Wasn’t this all incredibly cynical?

I think I’d use the word practical. The Soviets were there anyway and the Baltic States had been historically part of the old Czarist Russia. So Roosevelt was more or less willing to let them go back into the Communist system. Poland was right on the doorstep of Russia. Roosevelt didn’t like the Polish government in exile, and neither did Churchill, and they knew that there wasn’t much they could do once the war ended. What they were trying to do was placate Stalin so he wouldn’t get greedy and grab all of Eastern Europe, which, in fact, he did. They hoped that because he was an ally he would restrain himself. But he didn’t.

Churchill’s famous Iron Curtain speech showed that he believed Communism would be a greater threat even than Nazi Germany. He was right, wasn’t he?

Yes. He’d been on to Stalin from the beginning and he did not trust the Communists at their word. Roosevelt was [laughs] more ambivalent. He said to the New York Times reporter that Mr. Stalin was not in charge in the Kremlin. It’s an interesting statement because it shows that Roosevelt really wasn’t sure what was going on. Churchill was.

Today, the world is rife with conflict. Yet our leaders seem like very small compared with, say, Churchill or Roosevelt. What lessons can they give us about leadership for today’s world?

They were very strong leaders but they were also men. They made mistakes. We look at them now in the spotlight of history as giants. I think Churchill was seen as a giant in his time. Americans were a little more ambivalent about Roosevelt because he’d been in power for a long, long time. No one knows what the Russian people thought of Stalin because they didn’t take polls. [Laughs]

This interview was edited for length and clarity.