President Donald Trump has until October 15 — this Sunday — to make a decision that could sabotage the nuclear deal with Iran.

Under the Iran Nuclear Agreement Review Act (INARA), passed by Congress in May of 2015, the president must certify that Iran is in compliance with the deal’s terms every 90 days. The past two times this has come up in the Trump administration, in April and July, the president has reluctantly certified Iran’s compliance.

But this time around, things appear to be different. Trump, who viscerally hates having to approve something he’s called “the worst deal ever,” is reportedly set to decertify it. If these reports are correct — and you can never really be sure with this White House — then Trump will be setting off a major foreign policy crisis.

Decertification doesn’t, on its own, end the nuclear deal. But it would formally begin a process through which Congress could quickly reimpose sanctions on Iran that were lifted under the terms of the agreement. If the Republican-controlled body did choose to do that, it would pretty much guarantee that the deal falls apart, as Iran would have little reason to stay in it if sanctions are being put back in place.

The point is that this decision is a very big deal — one that has some experts on Iran really spooked.

“Decertification corrodes the legitimacy of the deal,” says Suzanne Maloney, deputy director of the foreign policy program at the Brookings Institution. “[It] will slowly collapse.”

What follows is a guide to the looming crisis — what decertification means, the legal and policy implications of Trump’s decision, and why the deadline has experts like Maloney so worried.

Iran is complying with the deal — and Trump’s top advisers know it

The nuclear deal between the US and Iran — or, more precisely, the US, Iran, China, France, Russia, the UK, and Germany — is a very long and technically complicated document. But its basic terms can be boiled down to a very simple transaction: Iran agrees to strict limits on its nuclear program and, in exchange, the other five countries relax sanctions imposed on Iran as punishment for its nuclear activities.

This agreement, formally named the Joint Comprehensive Plan of Action (JCPOA), appears to be working basically as intended.

The International Atomic Energy Agency (IAEA) has repeatedly confirmed that Iran has complied with the deal’s restrictions, such as the requirement that Iran dismantle thousands of centrifuges (devices that can be used to create weapons-grade uranium). This has moved Iran further away from being able to build a bomb, thus avoiding the horrible choice of either allowing Iran to become a North Korea-style nuclear power or launching a bloody war to try and stop it.

“By any objective measure, we’re not seeing any violations of the deal,” says Richard Nephew, an expert at Columbia University who worked on negotiations with Iran in the Obama White House between 2011 and 2013.

So if Iran is holding up its end of the deal, how can Trump refuse to certify its compliance? If you take a close look at the text of INARA, you’ll see that it requires the president to certify something else on top of technical compliance with the deal’s terms.

Specifically, it requires Trump to certify that “suspension of sanctions [is] appropriate and proportionate to the specific and verifiable measures taken by Iran with respect to terminating its illicit nuclear program” as well as “vital to the national security interests of the United States.”

This is much squishier language, as what’s in America’s interests is a subjective judgment. This provision gives Trump full discretion to decline to certify under INARA. “They could make whatever case they’d like,” says Michael Singh, a senior fellow at the Washington Institute for Near East Policy,

This, of course, raises a broader question: Why would Trump want to decertify Iran if it is, in fact, complying with the deal?

The case against recertifying the deal has instead been made by a handful of administration officials, most notably UN Ambassador Nikki Haley. They point to Iran’s legitimately troubling actions — such as its ballistic missile tests and support for terrorist organizations like Hezbollah — as evidence that it can’t be trusted. None of these activities, not even missile testing, is prohibited by the deal — yet Trump often cites them as evidence that Iran is not abiding by the “spirit” of the agreement.

“Iran’s leaders want to use the nuclear deal to hold the world hostage to its bad behavior,” Haley said in a September 5 speech. “It is this unwillingness to challenge Iranian behavior, for fear of damaging the nuclear agreement, that gets to the heart of the threat the deal poses to our national security.”

But the majority of Trump’s national security cabinet do not share this view. National Security Adviser H.R. McMaster, Secretary of Defense Jim Mattis, and Secretary of State Rex Tillerson all appear to oppose decertifying Iran. When Mattis was asked during an October 3 Senate hearing whether he thinks staying in the deal is in US national security interests, his answer was simple: “Yes, senator, I do.”

But ultimately, the decision doesn’t rest with Haley or Mattis or anyone else. The final call is made by the president, and thus hinges on President Trump’s worldview and idiosyncrasies.

Why Trump wants to decertify anyway

The president himself has not publicly offered a detailed policy case for decertification beyond general calls to renegotiate the deal’s terms, nor are there any reports of him doing so privately. Experts don’t see any signs that he’s particularly well-versed in the arguments.

“I don’t think anyone actually thinks he knows anything about the particularities of this agreement,” says Sarah Kreps, a professor at Cornell University who studies arms control agreements.

Reports from inside the White House instead emphasize Trump’s feelings about the agreement. When Trump certified the deal back in July, the Weekly Standard’s Stephen Hayes and Michael Warren report that Trump was “angry” and “irritated” about being pushed to do so by Mattis and Tillerson. Politico’s Eliana Johnson reports that Trump “unleashed a tirade” at his aides during that meeting, and was only convinced to certify Iranian compliance after an hours long argument that one aide described as “knock-down, drag-out fight.”

This emphasis on the president’s feelings make sense. Trump has spent years bashing the deal as a disaster, labeling it (among other things) an “embarrassment” and catastrophe.” In 2015, he gave the lead speech at an anti-deal rally in Washington, in which he said, “I've never seen something so incompetently negotiated.”

Being forced to repeatedly certify that his past rhetoric was misleading — that the Iran deal is, in fact, working as intended — is a kind of personal humiliation. Decertifying is way to avoid that, and to bring the administration in line with the past several years of Trump’s rhetoric.

“He seems to genuinely believe that you can get some kind of better arrangement with the Iranians despite the fact that I suspect he doesn’t actually know what’s in the deal,” Maloney says. “Fundamentally, his determination to go after the deal is as much personal as anything else.”

Jeffrey Lewis, a non-proliferation expert at the Middlebury Institute of International Studies, had a more colorful way of putting this point when we spoke in September.

“[Recertifying the deal] makes him feel like a cuck every 90 days,” said Lewis.

What decertification actually does

The sense from inside the administration is that, this time, no amount of pressure from Trump’s aides will budge him. The Washington Post, Politico, and the Weekly Standard have all reported in the past few weeks that Trump has made up his mind and will not certify Iran’s compliance by the October 15 deadline mandated by INARA.

If Trump does refuse to certify by Sunday, the Iran deal won’t immediately collapse. Decertification doesn’t mean the US is withdrawing from the deal.

The nuclear agreement itself doesn’t require the US president to officially declare Iran is in compliance every 90 days. That requirement is a matter of US law, something that Congress put in place as a way to assert its oversight powers over the agreement.

Yet that doesn’t mean decertification is irrelevant to the agreement’s fate. Far from it.

If the president declines to certify the Iran deal, the law kicks the issue back to Congress. Specifically, it provides Congress with a way to fast-track legislation within the next 60 days reimposing the sanctions on Iran that were suspended as part of the deal. A bare majority vote in both Republican-controlled chambers would be enough to reimpose sanctions, as the procedures for a sanctions vote set up under INARA do not give Democrats an opportunity to filibuster.

This is by far the most significant immediate consequence of decertification: Giving Congress the ability to quickly pass legislation that would almost certainly destroy the deal.

However, decertifying doesn’t force Congress to hold a vote on new sanctions — it just gives them the ability to do so quickly if they so choose. And it seems like the Trump administration will actually oppose the reimposition of sanctions. Four people familiar with the administration’s thinking told the Washington Post’s Anne Gearan and Karoun Demirjian that “Trump would hold off on recommending that Congress reimpose sanctions.”

If that’s right, then it kind of tips the administration’s hand here. It would suggest that the goal isn’t to use the October 15 deadline as an opportunity to blow up the deal. Instead, it would seem that the move would be mostly symbolic: An opportunity for the president to stop having to embarrass himself every three months in a way that doesn’t actually have any immediate practical consequences.

“I get to say, if I’m Trump, that this deal is not in the US national interests — which is all I want to say,” Nephew guesses.

There’s no guarantee this balancing act — assuming this is what Trump ends up deciding on — is sustainable. For one thing, Congress might choose to reimpose sanctions using its authority regardless of what Trump recommends. There’s deep loathing for the deal among Republicans in Congress, and it’s possible that the issue could take on a life of its own.

For another, the political pressure this decision creates might push Trump to torpedo the deal unilaterally.

There are actually a separate set of deadlines, entirely independent of INARA, for waiving a slew of sanctions on Iran. These come up every few months as well, and need to be waived each time if the US wants to uphold its end of the bargain with Iran.

For example, the 2012 Iran Freedom and Counter-Proliferation Act (IFCA) prohibits Americans from doing business with Iran’s leading energy, shipbuilding, and shipping companies. Under the terms of the Iran deal, the US is required to lift these sanctions — yet Congress did not repeal IFCA after the agreement was signed. Instead, President Barack Obama used authority provided by IFCA itself to waive its sanctions after the Iran deal came into effect. Each waiver lasts for 180 days; the deadline for the next IFCA waiver is January.

Now, imagine that Trump is weighing the decision to issue another IFCA waiver in a world where he’s said that keeping sanctions off Iran is not in the US national interest. There could be a lot of pressure for him from conservatives to reimpose sanctions unilaterally, the clearest and simplest way to detonate the deal.

“None of those [sanctions waivers] involved any legislation by Congress,” explains Elena Chachko, a doctoral student at Harvard Law School and contributor to the national security blog Lawfare. “If [Trump] wanted to, he could basically stop implementing the agreement.”

The bottom line, then, is that decertification does not formally end the Iran deal — but it creates serious threats to its continued existence.

If Trump chooses to decertify the deal, “We are on the very slippery slope that leads to JCPOA collapse,” says Nephew.

Could the US and Iran work together after decertification?

Ultimately, it’s difficult to say whether the administration will slide down Nephew’s slope.

Both he and Maloney are deeply skeptical that the agreement can survive decertification, even if Congress or the president choose not to immediately reimpose sanctions.

“The longevity of the deal comes into question if the United States is taking specific actions to undermine it, even decertification,” Maloney warns. “If the United States says we’re not willing, even for domestic political purposes, to acknowledge that this deal is still functional, it incites a process that will cause the deal to collapse over time.”

Some of the other experts I spoke to, like Cornell’s Kreps and the Washington Institute’s Singh, were more sanguine.

“If Congress can’t do anything then essentially this agreement perpetuates,” Kreps says. “The status quo, then, continues — and that’s the scenario through which we should not be terribly alarmed.”

What’s certain is that decertification would create a crisis in US-Iranian relations, as it would be the first concrete step the Trump administration has taken towards attempting to renegotiate or cancel the nuclear agreement.

The fate of the deal and, perhaps, Middle East stability more broadly would depend on how diplomats and political leaders in the two countries handled the fallout from decertification. Yet US-Iranian relations are rocky at the best of times, given the two countries have been at odds on issues across the Middle East for decades. Managing US-Iranian relations going forward is “not going to be easy, and it’s going to require a pretty deft diplomatic hand,” as Singh puts it.

Yet the Trump administration is a uniquely bad position to pull of this kind of diplomatic feat.

The administration’s attention is split by a pressing, extremely scary confrontation with North Korea. Vitally important diplomatic posts, like the assistant secretary of state supervising the Middle East, are currently unfilled. And the president and the secretary of state are at war with each other, with Trump publicly challenging Tillerson to an IQ test after reports filtered out that Tillerson had once called the president a “moron.”

What decertification would do, in short, is create yet another foreign policy mess at a time when the Trump administration is preoccupied and ill-prepared to deal with one. And the stakes — the preventing the rise of a hostile nuclear power in one of the world’s most unstable regions — are very high.