The three men were well-educated, ambitious, liberal leaders of their countries.
Much more than just natural allies, they were also friends who helped forge an end to the Second World War and sculpt the peace that followed it.
Canadian Prime Minister William Lyon Mackenzie King played a key role in a triumvirate that also included British Prime Minister Winston Churchill and U.S. President Franklin Delano Roosevelt, says Western history professor emeritus Neville Thompson in his newly published book, The Third Man.
As the world marks 76 years since the end of war in Europe, The Third Man provides new insights into a pivotal partnership at a key time in world history.
History professor emeritus Neville Thompson
Thompson's work - including a deep dive into the 30,000 pages of King's private, contemporaneous diaries - reveals for the first time the trio's warm, decades-long relationship.
King knew both the two other men better than they knew each other, Thompson said in an interview.
"Roosevelt and Churchill, they were both very gregarious, but they had to be careful what they said, and to whom they said it. And King was a good listener. He wasn't always trying to interject his own views, trying to dominate the conversation."
King had been a national leader for far longer than the other two, Thompson added. "When Churchill and Roosevelt talked to him, they were not talking as they might do with a friend who is in some other line of business, or even another minister; they were talking to someone who was also leading an important nation."
King wrote down their conversations in his diary, which he had originally intended should be destroyed.
Negotiations and meetings, disagreements, mutual aid, personal and professional observations, the men's views on Josef Stalin and Charles de Gaulle - all found a place in King's voluminous notes.
The book is not a hagiography, and Thompson notes the three men's biases and shortcomings, including the indefensible refusal by Canada and the U.S. to accept a shipload of Jewish Europeans fleeing Nazi Germany on the eve of the war.
But Thompson also resurrects the stature of King as intermediary on the world stage at a time when the architect nations of peace were Britain, the U.S. and Canada.
Cover of The Third Man, by Neville Thompson, published by Sutherland House press
He notes how the three were planning for a more stable post-war world even as they planned the D-Day landings at Normandy in 1944.
Churchill didn't get the post-war British Empire unity he sought; nor did Roosevelt found the global enforcer organization he wanted.
"But their idealism does some great credit," Thompson said. "I mean, they were not just fighting to defeat Germany, Italy and Japan - they genuinely wanted a better world. It didn't work out the way they thought. On the other hand, the world after the Second World War was, in general, better than it was after the First World War."
The Third Man: Churchill, Roosevelt, Mackenzie King and the Untold Friendships that Won WWII, is published by Sutherland House press. It is Thompson's fourth book.
