As for Iran, a deal with one hundred moving parts also serves it well. “The Iranians will cheat the way they always cheat, which is incrementally, not dramatically,” notes sanctions expert Mark Dubowitz of the Foundation for Defense of Democracies. “Sooner or later, we’ll spot a potential violation and get into a debate about forensics: Are the Iranians complying or not? This will eat up time before we even get to the political debate over what to do about it.”

That’s been the Iranian M.O. ever since their covert nuclear program was first exposed in 2002. We’ve been negotiating their noncompliance ever since. Why should a regime that has paid no price for dishonesty suddenly discover the virtues of honesty in a post-deal world?

Supporters of a deal offer three answers. One is that the sanctions relief the West will offer in the deal can always be reversed in the event Iran cheats. “We can crank that dial back up,” as Mr. Obama said about sanctions last year. They also argue that what Iran seeks is to become, in the Bismarckian sense, a “satisfied power,” one that achieves its goals of diplomatic normalization, economic prosperity and nuclear pride—but also knows its limits.