Of all the arguments for the Trump administration to honor the nuclear deal with Iran, none was more risible than the claim that we gave our word as a country to keep it. “Our?”

The Obama administration refused to submit the deal to Congress as a treaty, knowing it would never get two-thirds of the Senate to go along. Just 21 percent of Americans approved of the deal at the time it went through, against 49 percent who did not, according to a Pew poll. The agreement “passed” on the strength of a 42-vote Democratic filibuster, against bipartisan, majority opposition.

“The Joint Comprehensive Plan of Action (J.C.P.O.A.) is not a treaty or an executive agreement, and it is not a signed document,” Julia Frifield, then the assistant secretary of state for legislative affairs, wrote then-Representative Mike Pompeo in November 2015, referring to the deal by its formal name. It’s questionable whether the deal has any legal force at all.

Build on political sand; get washed away by the next electoral wave. Such was the fate of the ill-judged and ill-founded J.C.P.O.A., which Donald Trump killed on Tuesday by refusing to again waive sanctions on the Islamic Republic. He was absolutely right to do so — assuming, that is, serious thought has been given to what comes next.

In the weeks leading to Tuesday’s announcement, some of the same people who previously claimed the deal was the best we could possibly hope for suddenly became inventive in proposing means to fix it. This involved suggesting side deals between Washington and European capitals to impose stiffer penalties on Tehran for its continued testing of ballistic missiles — more than 20 since the deal came into effect — and its increasingly aggressive regional behavior.

But the problem with this approach is that it only treats symptoms of a failure for which the J.C.P.O.A. is itself a major cause. The deal weakened U.N. prohibitions on Iran’s testing of ballistic missiles, which cannot be reversed without Russian and Chinese consent. That won’t happen. The easing of sanctions also gave Tehran additional financial means with which to fund its depredations in Syria and its militant proxies in Yemen, Lebanon and elsewhere. Any effort to counter Iran on the ground in these places would mean fighting the very forces we are effectively feeding. Why not just stop the feeding?

Apologists for the deal answer that the price is worth paying because Iran has put on hold much of its production of nuclear fuel for the next several years. Yet even now Iran is under looser nuclear strictures than South Korea, and would have been allowed to enrich as much material as it liked once the deal expired. That’s nuts.

Apologists also claim that, with Trump’s decision, Tehran will simply restart its enrichment activities on an industrial scale. Maybe it will, forcing a crisis that could end with U.S. or Israeli strikes on Iran’s nuclear sites. But that would be stupid, something the regime emphatically isn’t. More likely, it will take symbolic steps to restart enrichment, thereby implying a threat without making good on it. What the regime wants is a renegotiation, not a reckoning.

Why? Even with the sanctions relief, the Iranian economy hangs by a thread: The Wall Street Journal on Sunday reported “hundreds of recent outbreaks of labor unrest in Iran, an indication of deepening discord over the nation’s economic troubles.”

The regime might calculate that a strategy of confrontation with the West could whip up useful nationalist fervor. But it would have to tread carefully: Ordinary Iranians are already furious that their government has squandered the proceeds of the nuclear deal on propping up the Assad regime. The conditions that led to the so-called Green movement of 2009 are there once again. Nor will it help Iran if it tries to start a war with Israel and comes out badly bloodied.

All this means the administration is in a strong position to negotiate a viable deal.

But it missed an opportunity last month when it failed to deliver a crippling blow to Bashar al-Assad, Iran’s puppet in Syria, for his use of chemical weapons. Trump’s appeals in his speech to the Iranian people also sounded hollow from a president who isn’t exactly a tribune of liberalism and has disdained human rights as a tool of U.S. diplomacy. And the U.S. will need to mend fences with its European partners to pursue a coordinated diplomatic approach.

The goal is to put Iran’s rulers to a fundamental choice. They can opt to have a functioning economy, free of sanctions and open to investment, at the price of permanently, verifiably and irreversibly forgoing a nuclear option and abandoning their support for terrorists.

Or they can pursue their nuclear ambitions at the cost of economic ruin and possible war. But they are no longer entitled to Barack Obama’s sweetheart deal of getting sanctions lifted first, retaining their nuclear options for later, and sponsoring terrorism throughout. Trump’s courageous decision to withdraw from the nuclear deal will clarify the stakes for Tehran. Now we’ll see whether the administration is capable of following through.