It’s official: President Trump is pulling the United States out of the Iran nuclear deal.

Trump’s Iran deal announcement was made in a Tuesday afternoon White House address, in which he announced that he would reimpose sanctions on Iran’s oil sector that had been lifted as part of the agreement. This put the US in violation of its obligations under the agreement, and thus constitutes a unilateral American withdrawal from the deal.

“We cannot prevent an Iranian bomb under the decaying and rotten structure of the current agreement,” the president said in justifying his decision. “Therefore, I am announcing today that the United States will withdraw from the Iran nuclear deal.”

The problem, though, is that the deal wasn’t “rotten”: The best evidence we have suggests Iran was actually complying with the deal. Iran has dismantled a huge portion of its nuclear program and given international inspectors wide latitude to make sure it isn’t cheating; the country is significantly further from a nuclear weapon than it was when the deal came into force.

Now, American withdrawal doesn’t mean the deal is immediately dead. Technically, the nuclear deal is an agreement between Iran and what’s called the P5+1 (the US, Britain, France, Russia, China, and Germany) — which means the US leaving the agreement doesn’t end it. If the rest of the P5+1 keep their sanctions off, Iran may decide to continue to adhere to the deal’s restrictions even after the US pullout. That’s what Iranian President Hassan Rouhani said his country would do in a Monday speech.

But the truth is that nobody actually knows what the Iranians will do. It’s possible they stay in the deal; it’s equally possible that they slowly chip away at the deal’s restrictions or even dash for a nuclear weapon. What’s more, the sanctions also could precipitate a major crisis with America’s European allies, as some of the sanctions could affect European companies that do business in Iran.

And that’s what makes this decision of Trump’s so scary. Trump took a major geopolitical flashpoint that had been contained and blew it wide open.

“Trump’s pullout [makes] this thing an agreement in name only,” says Hussein Banai, an expert on Iran at Indiana University Bloomington. “We don’t even know how to judge anything anymore.”

The Middle East just got a new crisis, and it’s one entirely of Trump’s making.

What Trump just did to the Iran nuclear deal

Prior to the Iran deal, the P5+1 had imposed a dizzying and complex set of international sanctions on Iran as punishment for its nuclear development. The basic gist of the deal was that the sanctions would be lifted in exchange for Iran agreeing to several serious restrictions on its nuclear development. Among the most important, the deal called for:

Reducing Iran’s stockpile of enriched uranium by 97 percent, and banning them from possessing any uranium potent enough that it could be used to fuel a bomb

Capping its number of nuclear centrifuges, devices used to enrich uranium, at roughly 5,000 — and only permitting it to use old, outdated, and slow centrifuges

Stopping Iran from operating its Arak facility used to make plutonium that could fuel a bomb

Permitting wide-ranging and intrusive IAEA inspections designed to verify that Iran isn’t cheating on any portion of the deal

These provisions, taken together, make it functionally impossible for Iran to acquire a nuclear weapon so long as they are in place. And the IAEA has repeatedly confirmed that Iran is complying with all of its obligations under the deal.

“With the deal in place, it would be extremely difficult for Iran to build the bomb without being detected — and there would be a time frame that would allow the international community to respond,” James Acton, director of the Carnegie Endowment’s Nuclear Policy Program, tells me.

But the whole point of the deal is a quid pro quo: Iran agrees to nuclear restrictions in exchange for the financial benefits of sanctions relief, which would amount to billions and billions of dollars. What Trump did today was reimpose a major portion of the pre-deal sanctions regime — something called “secondary sanctions” targeting Iran’s oil sector.

Secondary sanctions don’t punish Iran directly, instead targeting international banks that do business with Iran’s oil sector. Hence why they’re “secondary”: Instead of hitting the primary target, Iran, they cut off access to US markets for third parties that want to work with Iran. In effect, it forces foreign countries into a choice between importing large amounts of Iranian oil or doing business with the United States. Since America is the world’s largest economy, it’s not exactly a hard choice — and thus would end up punishing close US allies that want to do business with Iran.

Reimposing these sanctions puts the US in clear violation of its obligations under the deal, thus effectively withdrawing America as a participant — and significantly reducing Iran’s incentive to stay in.

But the deal isn’t dead, at least not yet.

If the rest of the countries don’t reimpose their own sanctions, Iran may very well end up still having more access to international markets than it had before the deal was inked. It could thus still decide to stay in the deal, rather than kicking out inspectors or restarting large-scale uranium enrichment, in order to avoid angering these other parties — all of which opposed Trump’s decision.

But we don’t know that the Iranians will do that for sure. They may decide that without the US, the deal isn’t really worth it. As a result, the issue of Iran’s nuclear program — whether or not a rogue regime will attempt to pursue a nuclear bomb — is now all of a sudden back on the table.

Why Trump withdrew from the Iran deal — and what could happen next

What’s weird about Trump’s decision is that he presented no evidence that Iran wasn’t complying with its obligations. Neither the president nor the foreign countries that support his decision, chiefly Israel and Saudi Arabia, have presented any evidence that Iran is technically in violation of the deal.

Instead, the president’s case revolved around perceived defects in the deal itself. The first one is that the deal isn’t entirely permanent; the restrictions on Iran’s nuclear program start to relax about 10 years after the deal was signed (though the agreement not to build a nuclear weapon is permanent). The second is that the deal didn’t cover other problematic things Iran was doing, including ballistic missile development and its support for violent militias around the Middle East (like Hezbollah in Lebanon).

French President Emmanuel Macron urged Trump to try to open up new negotiations on these issues while staying in the deal, thus keeping Iran’s nuclear program in check for now while dealing with these other issues. But Trump rejected that approach, preferring instead to withdraw from the deal entirely. Trump has said he’s open to negotiating a newer, tougher deal, but there’s little chance that Europe and Iran — which spent years negotiating the current deal, and believe that it’s working — would agree to do so.

It’s hard to understand the motivation for doing this without looking beyond policy and into politics. During the 2016 presidential campaign, Trump repeatedly called the Iran deal “the worst deal ever,” once giving the headline speech at an anti-deal rally. And a priority for this administration has been tearing down accomplishments of the prior president, be they Obamacare or the Paris climate accord; the Iran deal fits squarely under this umbrella.

Trump’s recent Cabinet reshuffle also made his decision seem inevitable. The president fired Secretary of State Rex Tillerson and National Security Adviser H.R. McMaster, both of whom thought the US should stay in the deal, and replaced them with Mike Pompeo and John Bolton, respectively. Both Pompeo and Bolton are Iran hawks who have long opposed the deal. In effect, Trump was surrounded by yes men while he made one of the most consequential decisions of his presidency.

Whether the logic was principally geopolitical or plain old political, the decision remains the same: America is out of the Iran deal. Now the question is what comes next.

Part of this hinges on how strictly Trump chooses to enforce the sanctions he’s reimposing. The Office of Foreign Assets Control (OFAC) is the branch of the Treasury Department responsible for implementing US sanctions; it has a lot of leeway to shape which international companies will be hit by secondary sanctions and how strict those sanctions will be. The less stringent OFAC’s guidance is, the less the new sanctions will hurt Iran.

Europe is unlikely to impose its own sanctions on Iran now, given that it believes the deal is working. If the US weren’t too strict about its secondary sanctions, Iran’s most important trading relationships would remain intact so long as Tehran adhered to the deal’s nuclear limits — giving it more incentive to adhere by them.

But if the secondary sanctions are implemented very strictly, and European firms stop doing business with Iran to avoid the risk of American punishment, then Iran would have little reason to stay in the deal. The details of Trump’s announcement aren’t just details; in some ways, they’re the most important part of the entire thing. And his announcement did little to clarify them.

Ultimately, though, no experts are really optimistic that the deal can survive this in the long run. US withdrawal creates too much uncertainty, and gives Iran too much of an incentive to try to push the limits of what it can get away with, for the deal to stay in place indefinitely. Even if it doesn’t collapse today, and Iran pledges to stay in, the situation is just not stable.

“I think we get into a cycle of death by a thousand cuts,” Acton says. “The US does a step to bring it into noncompliance with the deal. If diplomacy with Iranians fails to keep them in the terms, maybe they take a little step ... and then we go back and forth until the deal is dead.”

And if the deal does, in fact, fall apart — well, then Donald Trump will face a stark choice: Either let Iran advance toward the capacity to build a nuclear bomb or go to war to try to stop it. That’s the awful decision the Iran deal was designed to forestall, but it may very well be our future.