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Upon entering office, Barack Obama knew little about foreign policy. But then neither did Vice President Harry S. Truman when Franklin Delano Roosevelt died suddenly on April 12, 1945.

President Obama often invokes the supposed mess abroad—especially in Iraq and Afghanistan—left to him by George W. Bush. But Mr. Obama's inheritance is mild compared to the myriad crises that nearly overwhelmed the rookie President Truman.

All at once Truman had to finish the struggle against Hitler, occupy Europe, and deal with a nominally allied but increasingly bellicose and ascendant Soviet Union. Within months of taking office he had to make the awful decision to drop atomic bombs on Imperial Japan.

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Joseph Stalin, Harry Truman and...