“Idon’t give a shit about NATO.” Thus did former President Donald Trump once express his feelings about America’s oldest and strongest military alliance. Not that this statement, made in the presence of John Bolton, the national security adviser at the time, came as a surprise. Long before he was a political candidate, Trump questioned the value of American alliances. Of Europeans, he once wrote that “their conflicts are not worth American lives. Pulling back from Europe would save this country millions of dollars annually.” NATO, founded in 1949 and supported for three-quarters of a century by Democrats, Republicans, and independents alike, has long been a particular focus of Trump’s ire. As president, Trump threatened to withdraw from NATO many times—including, infamously, at the 2018 NATO summit.

But during Trump’s time in office, the withdrawal never happened. That was because someone was always there to talk him out of it. Bolton says he did; Jim Mattis, John Kelly, Rex Tillerson, Mike Pompeo, and even Mike Pence are thought to have done so too.

But they didn’t change his mind. And if Trump is reelected in 2024, none of those people will be in the White House. All of them have broken with the former president, in some cases dramatically, and there isn’t another pool of Republican analysts who understand Russia and Europe, because most of them either signed statements opposing him in 2016 or criticized him after 2020. In a second term, Trump would be surrounded by people who either share his dislike of American security alliances or don’t know anything about them and don’t care. This time, the ill will that Trump has always felt toward American allies would likely manifest itself in a clear policy change. “The damage he did in his first term was reparable,” Bolton told me. “The damage in the second term would be irreparable.”

Institutionally, and maybe even politically, leaving NATO could be difficult for Trump. As soon as he announced his intentions, a constitutional crisis would ensue. Senate approval is required for U.S. treaties—but the Constitution says nothing about congressional approval for withdrawal from treaties. Recognizing this gap in the law, Democratic Senator Tim Kaine and Republican Senator Marco Rubio introduced legislation, which has already passed the Senate, designed to block any U.S. president from withdrawing from NATO without two-thirds Senate approval or an act of Congress. Kaine told me he feels “confident that the courts would uphold us on that and would not allow a president to unilaterally withdraw,” but there would certainly be a struggle. A public-relations crisis would unfold too. A wide range of people—former supreme allied commanders, former chairmen of the Joint Chiefs of Staff, former presidents, foreign heads of state—will surely rally to make the case for NATO, and very loudly.