As of early March, 39 Republican members of the House and four senators had already indicated they would retire or resign before Congress begins a new session next year—and there may be more. This isn’t particularly noteworthy; waves of retirements often happen two years into a new presidency. But it is significant which Republicans are leaving, and their departure tells an important story about the state of the American right.

The list includes the chairman of the Senate Foreign Relations Committee, Tennessee Senator Bob Corker, who first announced in September that he would not run for reelection. Since he arrived in Washington in 2007, Corker has been one of the most vocal Senate Republicans on foreign policy, working to regulate Iran’s nuclear arsenal and telling The New York Times in October that Trump was putting the United States “on the path to World War III.” House Foreign Affairs Chairman Ed Royce, who has loudly called for stricter sanctions on Russia and Iran, is also stepping down, after 13 terms in the House; and although John McCain, who has earned a reputation for principled, and at times even bipartisan, foreign policy expertise, still has four and half years left in his current term, he is in poor health. So the real story of the Republican resignations is that the GOP is losing some of its last few members who believe in thoughtful (if conservative) internationalism, trusting in diplomacy, arms-control deals, and alliances like NATO. It is a hollowing out that could radically reshape how foreign policy is forged in Washington.

It’s clear that Republican leaders are worried: In February, they reportedly urged Corker to reconsider retiring because, as Politico wrote, they wanted to “preserve waning foreign policy experience in the GOP.” (Corker wasn’t swayed.) But the problem extends beyond experience. As the old guard of globalists departs Congress, the only congressional Republicans left with the foreign policy credibility to check Donald Trump are those who feel he isn’t warlike enough: Tom Cotton, Marco Rubio, Lindsey Graham—ultrahawks eager for more American military intervention abroad. “Most of them are conditioned to distrust diplomats and to view diplomacy as a waste of time,” Daniel Larison, a senior editor at The American Conservative, told me. Trump might seem foolish for his dismissal of the Paris climate accord and the Iran nuclear deal, but he’s merely echoing the sentiments of his hotheaded congressional GOP allies.

Republicans are losing some of their last members who believe in thoughtful internationalism, diplomacy, arms control, and alliances like NATO.

It’s a remarkable shift. Twenty years ago, Corker, Royce, and McCain were the norm in Republican politics. Ever since the late 1940s, when Arthur Vandenberg, a Michigan Republican who spent 22 years on the Senate Foreign Relations Committee, broke ranks with his party and became a powerful advocate for the Truman Doctrine, NATO, and the Marshall Plan, there’s been an internationalist, prudential wing of the GOP that believed in bipartisan statesmanship. The Cold War had forced Congress to rely on regional experts who understood alliances and foreign cultures, as well as arms-control experts fluent in the language of deterrence theory. Even Ronald Reagan, as hawkish as his rhetoric was, appreciated the dangers that nuclear war presented and, in 1987, negotiated the Intermediate-Range Nuclear Forces Treaty, which led the United States and the Soviet Union to eliminate thousands of missiles. To help craft the accord, he turned to formidable figures like Paul Nitze, former secretary of the Navy and longtime informal adviser to presidents, to gather support in the Senate. Think tanks like the RAND Corporation and the Brookings Institution were on call by both parties to guide lawmakers. And their experts anchored this internationalist worldview in the minds of Republican politicians for decades.

Republican senators like Richard Lugar and Chuck Hagel were heirs to this philosophy, as were McCain, Royce, and Corker. But with the end of the Cold War, their worldview became increasingly obsolete. You can’t really negotiate with Al Qaeda or ISIS the way you could with the USSR or Red China. Instead, U.S. foreign policy has become, in some ways, a matter of where to send the troops and drones. And with radical congressional leaders like Newt Gingrich and Paul Ryan snatching control of the party, aided by mass movements like the Tea Party, Republican moderates of any sort have started to feel unwelcome in Washington. Lugar fell to a Tea Party challenger in Indiana’s 2012 Republican primary; Hagel decamped from the Republican Party altogether, joining Obama’s White House as defense secretary the following year.