So why should the P5+1 nations—the five permanent members of the U.N. Security Council (the United States, Great Britain, France, Russia, and China) plus Germany—pursue this deal, despite the uncertainties?

The main reason is that it is a profoundly good deal; there has never been a nuclear deal, with any country, that is so comprehensively restrictive. Israeli Prime Minister Benjamin Netanyahu urged the U.S. Congress to demand “a better deal,” but his definition of such a deal—one that bans uranium enrichment, dismantles all its facilities, and insists on a drastic change in Iran’s foreign policy—is unattainable, and, more to the point, he knows it.

Yes, this deal wouldn’t keep Iran from being a menace in Middle East politics, or from repressing its own people. But no arms control deal can aspire to do that. The U.S.-Soviet strategic arms treaties, signed throughout the Cold War, didn’t require the Soviet Union to disavow communism, end its support of Third World insurgencies, or institute Jeffersonian democracy—but the deals were still very useful. They capped, and in the later years reversed, the nuclear arms race; and they provided a forum for diplomacy, a cooling-off of the distrust and hatred, at a time when no other issues could have done so.