Obama often talks about moments in which U.S. leadership can “bend the arc of human history.” An Iran accord represents exactly such an opportunity, but it is also one of the most risky foreign policy gambles of his presidency.

The talks revolve around an issue — nuclear proliferation — that has been a major focus for Obama since he first arrived in Washington. As a senator, he called for a world without nuclear weapons. As president, his first foreign policy speech focused on the dangers that a terrorist group, such as al-Qaeda, might someday acquire a nuclear bomb.

“If we believe that the spread of nuclear weapons is inevitable,” he told a crowd of thousands in Prague’s main square, “then in some ways we are admitting to ourselves that the use of a nuclear weapon is inevitable.”

The Iran talks also reflect his abiding belief that the best way to change the behavior of hostile governments with spotty human rights records is not through isolation or the threat of military force, but by persistent engagement. In recent years, Obama has pushed to open up trade and diplomatic relations with countries such as Cuba and Burma.