Donald Trump, it appears, has never met an international agreement he wasn’t prepared to trash. On June 1, the president stepped to the podium in the Rose Garden and announced his decision to withdraw from the Paris climate accord. This followed on the heels of his reluctance to reaffirm America’s commitment to defend its NATO allies, an obligation dating back seven decades, and his decision to walk away from the Trans-Pacific Partnership, the largest multilateral trade deal in a generation. The next potential target on the president’s hit list: the Iran nuclear deal, an agreement he has called “the worst deal ever negotiated” and has pledged to dismantle.

The stakes could not be higher. The deal—officially known as the Joint Comprehensive Plan of Action—places significant and verifiable constraints on Iran’s nuclear activities, effectively blocking its pathway to an atomic bomb. If Trump exits the agreement, the prospects of a nuclear-armed Iran—or a major war to head off that outcome—would increase.

The Trump administration is in the midst of a comprehensive review of the Iran deal, and the conclusions are likely to be harsh. Indeed, in a notoriously factionalized administration, being hawkish on Iran is one of the few things that Trump officials seem to agree on.

Hostility toward the Islamic Republic starts at the top. Trump has consistently claimed that the nuclear deal requires U.S. taxpayers to provide Iran with a $150 billion “lifeline” in exchange for “nothing.” Both claims are false: The deal frees up around $60 billion of Iran’s own money, and the nuclear constraints are considerable. The president has also accused Iran of “not living up to the spirit” of the “terrible agreement,” even though his own State Department certified in April that Iran is complying with the deal. During his visit to Saudi Arabia in May, Trump singled out Iran for its support of terrorism, and even hinted at the need for regime change. And in June, after Islamic State militants attacked the Iranian parliament and the shrine of Ayatollah Khomeini, Trump noted that “states that sponsor terrorism risk falling victim to the evil they promote”—implying that Iran was asking for it.

Trump’s hard-edged perspective is shared across his administration. His national security advisor, Lt. Gen. H.R. McMaster, as well as key staffers on the National Security Council responsible for Middle East policy, forged their views of Iran at the height of the Iraq War, when U.S. troops were battling Iran’s Revolutionary Guard and its Shia militia proxies. Secretary of Defense Jim Mattis, another notable Iran hawk, commanded U.S. forces across the Middle East from 2010 to 2013—a time when his top job was preparing for a possible war with Iran to thwart its nuclear ambitions. This April, during a trip to Saudi Arabia, Mattis flatly stated: “Everywhere you look, if there’s trouble in the region, you find Iran.” Secretary of State Rex Tillerson has also expressed hard-line views, decrying the nuclear accord as a “failed approach” that does little to address the threat Iran poses across the Middle East.

So it would certainly come as no surprise if the Trump administration decides to walk away from yet another international accord. But even if Trump doesn’t torpedo the Iran deal directly, he could wind up scuttling the agreement by pushing Iran into a corner. In the coming months, the president is likely to embrace a much more aggressive posture toward Tehran, including more sanctions, more military exercises in the region, interdicting more Iranian vessels, selling more arms to Israel and Arab states, and taking direct action against Iran’s militant proxies—moves that could serve to escalate tensions and increase the likelihood of a military confrontation.

The gravest risks will arise as the campaign to defeat ISIS enters its final stage. Once U.S.-led forces push the jihadists out of Mosul and Raqqa, the two largest cities still under their control, Washington and Tehran could find themselves on a collision course over control of Iraq and Syria. Since mid-May, U.S. aircraft have carried out several strikes against Iranian-backed forces that were approaching At Tanf, a desert garrison for anti-ISIS forces along the Syria-Iraq border. Tehran hopes to build a “land bridge” linking Iran to its Hezbollah allies in Lebanon, traversing Iraq and Syria. For that reason, Iranian-backed militias are racing to secure key crossing points on both sides of the Syria-Iraq border. As U.S.-backed fighters approach ISIS’s last havens from the north and south, and Syrian regime and Iranian-backed forces approach from the west and east, the prospect of clashes between the United States and Iran will grow.

Any dustup in Syria could easily spill over into Iraq, where 6,000 U.S. troops operate in close proximity to tens of thousands of Shia militia fighters aligned with Iran. Since the counter-ISIS campaign began in 2014, these militias have avoided attacking U.S. forces because Washington and Tehran have been fighting the same enemy. But once Mosul falls, the common cause against ISIS will rapidly dissipate. And if the Iranians start taking casualties at the hands of U.S. forces in Syria or elsewhere in the region, Tehran could retaliate by unleashing Shia militias to target Americans. At that point, things could quickly get out of hand. In 2011, after Iranian-backed militias killed more than a dozen U.S. soldiers in Iraq, Jim Mattis, America’s commander in the Middle East at the time, recommended launching retaliatory missile strikes into Iran. In the end, he was overruled by President Barack Obama. But if Iran renews such attacks by its militia proxies, it’s easy to imagine Mattis making a similar recommendation to Trump.

To make things worse, two factors that helped defuse tensions with Iran during Obama’s tenure are now absent. First, the high-level diplomatic channel that Obama established with Iran no longer exists. In January 2016, when two U.S. patrol boats drifted into Iranian waters and ten American sailors were seized by Iran, urgent exchanges between Secretary of State John Kerry and Iranian Foreign Minister Javad Zarif averted a potential crisis. The sailors were returned within 24 hours, without a shot fired. In the absence of such a high-level channel, however, it is hard to imagine the Trump administration peacefully resolving a similar incident today.

Second, while Obama wanted to avoid a war with Iran, Trump may have a different political calculation. The one time Trump was almost universally viewed as “presidential” by the Washington establishment was after he ordered a missile strike on a Syrian airfield last April. Having internalized that lesson, Trump may be tempted to ratchet up hostility with Iran to distract from controversies over his ties to Russia and his failure to advance his policy agenda.

Whether or not Trump “rips up” the nuclear deal or launches an all-out war against Iran, rapidly escalating tensions could make the deal politically unsustainable in either country. And while his decision to withdraw from the Paris climate accord raises the long-term odds of an environmental catastrophe, exiting or undermining the agreement with Tehran could unleash a far more immediate disaster. With North Korea edging closer to an intercontinental ballistic missile, China expanding in the South China Sea, and Russia hacking U.S. elections, a nuclear crisis with Iran is the last thing the world needs. Yet Trump appears to be steering—or stumbling—in precisely that direction.