In principle, each of the eight parties to the July 2015 nuclear agreement with Iran can undo the deal by not living up to the obligations deriving from it. In practice, only two might actually do so: the United States and Iran. The focus has so far been almost exclusively on the possibility of Iran cheating, for which the accord includes a so-called snapback mechanism that would automatically reimpose economic sanctions. In contrast, the question of what would happen if the United States were to sneak out of the deal—that is, if the U.S. Congress were to override a presidential veto of a joint congressional resolution disapproving the agreement—has not been adequately discussed.

Based on the relevant U.S. legislation in force, Congress has the right to review the deal, to approve or disapprove it, and to overrule a presidential veto of a congressional decision if a sufficient number of members of Congress choose to do so. Yet the legislature should not take this step without full consideration and recognition of the international consequences of such action.

In other words, by choosing to pass the Iran Nuclear Agreement Review Act of 2015, Congress has put itself in a position in which it not only can judge the deal on its merits but also has to take into account the outcome of its own verdict. This was clear during the deliberations on the bill in the spring yet has somehow slipped from view in the heated debate that has followed the signing of the nuclear accord.

The consequences of Congress stopping the deal would be harsh for the United States and chaotic for international order. Ultimately, it would put into question the ability of the United States to lead the reshaping of the world order on Western terms, by alienating Washington’s European allies and allowing China and Russia to mock U.S. leadership, all while ceding the moral high ground to Tehran.

But let’s not immediately jump to these drastic predictions, as certainly many of the deal’s opponents would argue that it does not have to happen this way. Instead, let’s look at the process of congressional review in more detail.

The Mechanics of Congressional Review

Until September 17, 2015, Congress has the opportunity to vote on the Iran deal. Congress can approve or disapprove the agreement, or it could not vote on it at all. In the latter case (as well as in the case of an approval, which is unlikely given that Republicans control both houses and are against the deal), the president can fulfill the U.S. obligations under the deal and begin the process to “cease the application of”—that is, waive—U.S. sanctions against Iran. Such sanctions relief would only come into effect on implementation day after the International Atomic Energy Association (IAEA) has confirmed that Iran has fulfilled its up-front obligations to reduce its nuclear infrastructure. That could happen in early 2016.

If a resolution of disapproval passes both houses of Congress, the president has promised to veto it. Importantly, a veto-proof majority (two-thirds of votes cast in each chamber) does not mean that the deal is dead—yet. Such an outcome could just as well be an invitation to intense domestic deal making, namely to find ways to toughen U.S. policy on Iran outside the nuclear realm to make a compromise over the deal possible and avoid a veto override. The National Defense Authorization Act for Fiscal Year 2016 could be such a vehicle, for example to strengthen U.S.-Israeli military cooperation or U.S. ties with the Arab Gulf states, aimed at compensating for the sanctions relief Iran would receive.

Only if Congress successfully overrode the presidential veto would the legislature’s refusal of the accord actually stand. What then?

At least technically, this would still not be the end of the deal—as much as the naysayers might wish. This outcome would mean only that one of the eight parties has problems in implementing its part of the deal. As the agreement does not establish rules for national implementation processes, the mere fact that Congress disapproves the deal, and thus forbids the president from acting within the authority granted to him under the relevant sanctions legislation, would not constitute a legal violation of the deal itself. Only if and when U.S. President Barack Obama failed to waive U.S. sanctions as prescribed in the agreement would the United States be in breach of it.

Therefore, despite the likely international outcry that would ensue over a congressional no, the focus at that point should be on whether there is any way the administration could live up to its obligations in the face of Congress’s disapproval. The language of the review act is fairly watertight on this, stipulating that “action involving any measure of statutory sanctions relief by the United States . . . may not be taken” if Congress disapproves the deal.

So the question arises of what would happen if the president, fully aware of the consequences of a U.S. withdrawal from the deal, were to begin preparations for lifting sanctions regardless of congressional disapproval. Ultimately, this would set off a legal conflict that could only end with the Supreme Court deciding authoritatively on the administration’s prerogative in conducting and executing foreign policy. This is the question at the heart of congressional oversight of the Iran deal—and an issue that the review act merely papered over at the political level.

While in no way desirable, it is thus conceivable that, rather than delivering a clear no to the deal, Congress’s disapproval would set in motion a fierce domestic debate less about the merits of the agreement than about the administration’s authority to conclude it.

Few Fixes, Great Fallout at the International Level

Viewed bluntly from an international perspective, this is where the problem belongs: America’s inability to live up to an agreement concluded by the U.S. secretary of state as mandated by the U.S. president and commander in chief should not cause trouble for everyone else.

Faced with a congressional disapproval, the negotiation partners would probably still try to save the deal, which they have just agreed to be the best possible solution to the conflict over Iran’s nuclear program. The five permanent members of the UN Security Council plus Germany (P5+1) and the European Union would meet with Iran to discuss a way out of the impasse. Because the agreement itself would not yet be in force, this would not be a meeting of the Joint Commission foreseen to deal with such conflicts. However, there is very little that the other seven parties could do to preserve the deal if the United States decided not to stick to it.

A small possibility exists that Iran would fulfill its obligations for the up-front dismantling of its nuclear infrastructure so that the deal could move to the implementation phase. This would include a yes vote from the Iranian parliament, the Majlis, contrasting with the American no. Yet the government of Iranian President Hassan Rouhani would only do this to lay the blame squarely with the United States for not doing its bit. In that sense, Washington should fear such Iranian compliance more than the noncompliance some expect, as it would make the United States look even more like a spoiler.

More likely, however, Iran’s hardliners would urge the resumption of the country’s full nuclear program: installing new centrifuges; enriching uranium in greater quantities and to higher levels than under the current restricted terms; and continuing work on Iran’s plutonium-generating heavy-water reactor. This would probably not be a dash for the bomb—the Iranians would happily let the United States be the villain. Instead, Tehran’s activities would take place under regular pre-2013 IAEA inspection levels.

Ironically, the review act stipulates that a vote of disapproval also precludes any sanctions relief under the November 2013 interim agreement that led to the July 2015 comprehensive accord. This voids the possibility of the partners simply extending the interim agreement while negotiating a better deal. Unfettered by any restriction, Iran could shorten its breakout time—the period required to produce enough weapons-grade uranium for one nuclear weapon—to a few months, if not weeks. Determining if the country has reached the threshold for the bomb would be far more difficult to detect than with the agreement in place.

With Iran able to claim that its resumption of nuclear activities is a reaction to the U.S. withdrawal from the deal, the P5+1 coalition would begin to fall apart. China and Russia would turn against the United States, and the Europeans would not recover anytime soon from this punch in the face from their closest ally. While Washington could maintain its own sanctions and the EU might struggle to do the same, the rest of the world—on whose support the sanctions regime has rested—would resume trade with Iran. U.S. sanctions would still target companies in third countries doing business with Iran, but these measures would become less effective over time as they would lack international support. Thus freed to sell its oil, Tehran could enjoy both the money flowing in and the centrifuges spinning, all without the current restrictions under the interim accord, let alone the farther-reaching limitations that the comprehensive agreement puts in place.

The Europeans would be the worst off. A congressional no would not only destroy the diplomatic process they have nurtured ever since France, Germany, and the United Kingdom began negotiating with Iran in 2003. It would also rob them of any illusions about their closest ally at a time when support for U.S policy in Europe is already low. Would the Europeans follow the American lead in confronting Russia or the self-proclaimed Islamic State? Why should they? Would they agree to a transatlantic trade pact? Not if the U.S. Congress sets a precedent by renouncing a negotiated agreement.

Such reactions would not be rational, given that Europe still needs the United States for its own security and prosperity. Yet, these are the kinds of responses that an irrational provocation like congressional disapproval of the Iran deal—and the United States’ ultimate inability to implement its part of the agreement—might engender. As a consequence, each instance of transatlantic cooperation would have to be bargained and paid for, rather than taken for granted among friends. By the way, restarting nuclear negotiations in the hope of a better deal would not even be on the table after a snub from Congress, as the international partners would not be able to trust the United States to live up to its existing commitments.

China and Russia, for their part, would feel vindicated by a U.S. refusal to play by the rules. Given the goodwill they detected from the Iranians and the ill will displayed by the U.S. Congress, Beijing and Moscow would feel under no obligation to uphold the international sanctions regime. In the same vein, countries like South Korea, India, and probably Japan would see no reason to continue their freeze on imports of Iranian oil. Of course, congressional sanctions threatening all these countries would remain in place, but could the United States really enforce them against its close allies? Possibly it could, given the dominance of the dollar-denominated economy, but what price would the United States have to pay on all other fronts to literally buy acquiescence on the Iran issue?

More generally, such a move would hardly increase America’s standing and power in the world, let alone its ability to defend Israel in an increasingly hostile regional environment. The eagerness of most of the world to rehabilitate a rejected Iran would only be matched by its frustration at American sanctimoniousness.

A Choice for Congress

In the first U.S. Senate hearing on the Iran agreement, Bob Corker, chairman of the senate’s foreign relations committee, complained that with this deal, the administration had effectively turned the U.S. legislature into a pariah, instead of Iran. That’s patently untrue, given that it was Congress itself that fought for the right to approve the deal. It now has the opportunity to do so. If the legislature concludes that it does not want this agreement, it should be able to live with the consequences of this decision—and not be surprised if defying other global powers, including America’s closest allies, makes the United States the odd man out.

This certainly would not enhance U.S. national security, on which opponents of the deal claim to focus. Congress and its leadership are doing a disservice to the country by pretending that debate and scrutiny of the deal are possible without incorporating the damaging consequences of a no vote.