President Trump’s contempt for his predecessor’s chief foreign policy accomplishment, the Joint Comprehensive Plan of Action (JCPOA), or the nuclear deal with Iran, is no secret. On the campaign trail, he routinely called it one of the “worst” and “stupidest” agreements in history, and he pledged to dismantle it. Now, in his first speech before the UN General Assembly, Trump has suggested that if he has his way, the United States won’t continue to implement it.

During his speech, Trump described the deal as “one of the worst and most one-sided transactions the United States has ever entered into” and “an embarrassment to the United States.” And he pledged once again to take action on it, stating, “And I don’t think you’ve heard the last of it. Believe me.” The president justified his position, arguing, “We cannot abide by an agreement if it provides cover for the eventual construction of a nuclear program.” Of course, the JCPOA doesn’t provide “cover” for Iran’s nuclear program; its nuclear program is the very subject of deal. Nevertheless, the speech implied that the administration won’t be sticking to the agreement much longer.

In mid-October, Trump must announce his administration’s approach to the JCPOA. As part of the agreement’s implementation, the president must certify to Congress every 90 days that Tehran is (or isn’t) complying with its obligations under the agreement. If the president communicates to Congress that Iran isn’t complying, or simply fails to certify that it is complying, he kicks the ball over to Congress for new potential sanctions to be imposed on the country.

But so far, there’s no evidence to back Trump’s stance on the JCPOA and to justify his potential decertification or failure to certify the agreement. The International Atomic Energy Agency, a UN body tasked with monitoring Iran’s nuclear program, has issued several reports on the status of the nuclear deal’s implementation to date, stating that Tehran is upholding its end of the bargain. The United States’s allies and negotiating partners, the European Union and its member states, have echoed this view. On the day of Trump’s UN General Assembly speech, Vice President Mike Pence met with the EU’s High Representative for Foreign Affairs and Security Policy, Federica Mogherini. Mogherini chairs the joint commission that oversees the nuclear deal’s implementation. According to a press release put out by her staff, she told Pence that the agreement “is working, and that Iran is delivering on its commitments, as certified so far seven times by the International Atomic Energy Agency (IAEA). It is therefore important that a full and effective implementation of the JCPOA, in all its parts and by all parties, continues.”

Other European leaders have come forward in support of this position. French President Emmanuel Macron, for example, has become much more proactive in recent weeks, signaling to the United States that the agreement can’t be repealed and replaced. Speaking shortly after Trump at the UN General Assembly, Macron warned the United States that it’d lose confidence if it doesn’t abide by its commitments; he called repealing the deal reckless in the absence of alternatives. If America decides to unilaterally withdraw from the JCPOA while it’s working, undermining multilateral agreements and empowering Iranian hardliners, it’ll have to go it alone.

And the prospect of Trump withdrawing America from the nuclear deal isn’t just worrisome to Europeans.

Even critics of the nuclear agreement have expressed concerns about the possibility of a unilateral U.S. withdrawal without any clear evidence of Iranian noncompliance. For example, Senator Bob Corker, a vocal Iran hawk and opponent of the Obama administration’s Iran policy, has said that the White House shouldn’t tear it up. And even Trump’s own cabinet, including its many Iran hawks, such as Secretary of Defense James Mattis, are opposed to withdrawing from the process. After all, this isn’t just about the future of the nuclear deal or Iran, but about American credibility.

What does all this mean for the future of the JCPOA? Trump’s UN speech makes it seem increasingly likely that he won’t recertify Iranian compliance with the deal in October. Whether he will go so far as to all-out decertify it remains to be seen. But he may well kick the ball over to Congress, making it a key player in determining the agreement’s future.

For its part, Iran has signaled that it’ll continue to implement the agreement even if the United States withdraws, provided that its European partners stick to it. This has caused quite a stir in Iran. But Iran’s statements are designed to put the onus on the Europeans to preserve the future of the JCPOA regardless of the United States’ decision. As a result, it could ostensibly continue without America. But U.S. withdrawal wouldn’t come without implications.

First, continued implementation of the nuclear agreement without the United States will serve to undermine the country’s leadership in this process—and future ones, too. It’ll send a signal to the international community that such major endeavors don’t need America. And next time Washington tries to build a coalition and bring friends and foes to the table to tackle a security challenge, allies and adversaries alike will think twice before jumping on board. And sanctions, as a tool of foreign policy, will be weakened.

Second, without America on board, the international community will have a difficult time implementing the JCPOA. Indeed, investors and businesses will be more reluctant to engage Iran, making it more difficult for Iranians to collect the dividends of the deal—and, therefore, less enthusiastic about implementing it.

Lastly, American withdrawal without a substantive reason will empower Iranian hardliners. For decades, the Islamic Republic has tried to sell an anti-American ideology to the Iranian people. But its “death to America” chants have largely failed to bring the Iranian people on board. Trump, however, has been helping the regime make that case, through his anti-Iran rhetoric in Riyadh, his failure to send a sympathetic message to Iranians after the Islamic State’s attack in Tehran in June—and, now, through what seems to be an imminent withdrawal from the nuclear deal.