Why negotiate now? On Jan. 6, 2020, Iran is scheduled to take yet another step in rolling back its compliance with the 2015 nuclear accord that the Trump administration has left. This time it might resume enriching uranium to a 20 percent level, which is dangerously close to weapons grade. That could push European signatories to support a snapback of United Nations sanctions. Tehran has already threatened that it will counter such a step by withdrawing from both the nuclear deal and, more important, the Nuclear Nonproliferation Treaty –- steps that North Korea took before developing its nuclear arsenal.

Tehran could also continue to orchestrate attacks in the region that would not give the United States a clear case for war. Mr. Trump’s aversion to responding to such provocations has taught the Iranians that their risks of backlash are limited and manageable. Some in Tehran even argue that Iran could absorb any American military retaliation, and that the closer the United States gets to the threat of all-out war with Iran, the more it will consider returning to negotiations. As the economic situation in Iran worsens, the leadership may feel it has little to lose and little to fear — a dangerous combination.

The electoral calendars in the United States and Iran add another layer of complexity.