Because the Iran deal is an executive agreement, not a treaty, the new president has great latitude to alter or scrap it. But two can play that game: The Iranians are deeply unhappy with the accord, too, arguing that they never got the relief from sanctions that they were promised. Any effort to reopen the bargaining will also give Iran’s mullahs, military officers and conservatives a chance to alter the pact — or threaten to resume their race for nuclear capability.

To Mr. Trump, the Iran deal was not only misguided, but also badly negotiated. “They should’ve walked,” he said of Secretary of State John Kerry and his negotiating team. Mr. Trump said he would have left the negotiating room, doubled down on sanctions, and never agreed to give back billions of dollars, money that belonged to Iran and was frozen in American financial institutions.

But when pressed, he struggled to name any part of the deal he would have walked out of the negotiations to alter. With some prompting, he finally settled on a common critique: that after 15 years, Iran will be free to enrich uranium and reprocess plutonium again, in any quantity.

In “America First” terms, Mr. Trump’s decision about what to do with the Iran deal will be an early test of his willingness to act unilaterally. The nations that joined the United States in the negotiations — Britain, France, Russia and China — not only support the deal, but are rushing to take economic advantage of it by building commercial ties with Iran. If Mr. Trump wanted to abandon the deal or reimpose sanctions, they would almost certainly refuse to go along.

The Iranians hold a few cards, too. In January they shipped 98 percent of their nuclear fuel out of the country, disabled a plutonium reactor and took thousands of centrifuges, which enrich uranium, out of service. If the deal were to be declared dead, they would be free to re-create their nuclear infrastructure and rebuild their stockpile, now frozen until 2030. By Obama administration estimates, it would take about a year for them to produce enough new material for a weapon — longer to produce the weapon itself.

One option for Mr. Trump, advocated by many Republicans, is to simply reimpose sanctions on Iran for non-nuclear reasons, including its activities in Syria and its continued support of terrorism. The Iranians would say that violates the spirit of the agreement — and Iran’s leadership has already threatened that such action would nullify it.

Nuclear North Korea

It is hard to say which has sounded more confrontational in recent years about North Korea: the Obama administration, which has regularly warned of a “devastating response” to any North Korean use of its expanding nuclear arsenal, or the Republicans, who spent much of the Bush years plotting ways to make the country’s regime collapse.