What a strange debate. For weeks, Hillary Clinton and Bernie Sanders have been struggling over the future of their party and the allegiance of its youth. Yet in Thursday night’s PBS face-off, they spent an extraordinary amount of time quarreling over Henry Kissinger’s legacy and his role in Vietnam, Cambodia, and Mao’s China—a man, and controversies, that even the parents of many Sanders fans might barely remember.

It has become routine for Sanders to flay Clinton, time and again, for her mistaken vote in 2002 to authorize the use of force in Iraq, especially since he has no coherent ideas on what to do about the crises of today. But things took a wild turn in their latest duel when Sanders raked Clinton over the coals for boasting of the praise that she’d won from Henry Kissinger. “I will not take advice from Henry Kissinger,” Sanders pledged, denouncing him as “one of the most destructive secretaries of state in the modern history of our country.”

On this point, Sanders was right. Kissinger helped Richard Nixon step up the bombing in Vietnam, to no effect but massive death tolls, then acceded to a shameful way out and, in between, broadened the air war to Cambodia, which—as Sanders charged—destabilized its neutral government and spawned a power vacuum, which was filled by Pol Pot’s murderous Khmer Rouge.

It was nostalgia night for those who remember the campus teach-ins of the 1960s and ’70s and the critical scholarship in the decades that followed. Sanders might also have thrown in Kissinger’s role in the CIA plot to overthrow Chile’s Salvador Allende, his cynical leveraging of the atrocities in Bangladesh, and the list could go on.

But just as he was building steam, Sanders took a weird turn, denouncing Kissinger’s nearly undisputed diplomatic triumph—his opening to China, which Sanders described as leading to the loss of American jobs because of subsequent trade deals.

The befuddlement of the moment left an opening for Clinton’s most jabbing riposte of the night. “Journalists have asked who you do listen to on foreign policy,” she crowed, “and you have yet to answer that.” Sanders huffed back, “It ain’t Henry Kissinger”—which is fine, but who is it? Clinton replied quietly that she listens to many people.

Let’s look a little closer at Clinton’s touting of Kissinger, though. It was a strange moment indeed, in the New Hampshire debate on Feb. 5, when Clinton said, “I was very flattered when Henry Kissinger said I ran the State Department better than anybody had run it in a long time.” No doubt Sanders had been studying up for an answer, and waiting for an appropriate moment to pounce, ever since.

In an interview with USA Today back in 2014, Kissinger did indeed say of Clinton’s term as secretary of state, “She ran the State Department in the most effective way that I’ve ever seen.” The reporter asked if she’d done this more effectively than he had. “Yes,” he replied with a smile, “I was more chaotic.”

There is no question, Clinton jumps hoops over Sanders when it comes to foreign policy. Where he is eloquent on most other subjects, he’s clearly sweating when the debates turn to the world outside our borders. Hence his constant retreat to the 2002 vote on Iraq and now her presumed guilt by association for secret decisions made between 1969 and 1972.

Nonetheless, Clinton really should stop quoting the likes of Kissinger, and not just because of his record. There are basically two kinds of secretaries of state: those who see themselves as serving the president, and those who see themselves acting as emissaries to the White House on behalf of the diplomatic corps. Kissinger was very much in the camp of the former. The notion that, as secretary of state, he should have spent a single moment “running the State Department” would have struck him as a joke. From someone of Kissinger’s temperament, the compliment might well have been a bit backhanded.

Still, if foreign policy and national security were the deciding issues in this race, Clinton would have it wrapped up. The thing is, though, they’re not.

Read more of Slate’s coverage of the Democratic primary.