In a season full of comments we never thought we’d hear during a modern American presidential campaign, this one, spoken at the debate Tuesday night by of course Donald Trump, is arguably the most shocking: “I would be very, very firm with families. Frankly, that will make people think because they may not care much about their lives, but they do care, believe it or not, about their families’ lives.”

It’s not the first time Trump has said it, but it hasn’t gotten the focus it deserves. This idea of punishing or somehow threatening the family members of criminals has a name. It’s called collective punishment. And it has a history, which as you’d imagine is not pretty—think, oh, Stalin, for starters. And finally it has a status in international law. Under the Geneva Conventions, collective punishment is a war crime.

Collective punishment can take and has taken different forms. It doesn’t have to mean family members. In many cases it has meant the relocation/eradication of entire villages in response to rebellious or perceived treasonous acts by a few. It might also mean a kind of generalized and indiscriminate violence visited upon a population. Scholars debate, but surely Southerners would all agree, that William Tecumseh Sherman engaged in collective punishment during his infamous March to the Sea. You know, the one through that state, Georgia, where in the latest poll Trump holds a 27-point lead.

But in many cases, it does refer to families. Trump’s antecedents here are chilling. The Nazis used collective punishment against Poles and others who harbored Jews. The website of Yad Vashem tells the horrifying story of the Ulma family, who hid a Jewish family on their farm in 1942. They got ratted out, and the entire family, including six living children and one more in utero, was shot.

But Stalin was the master of collective punishment. It was for a time against the law in the USSR to be a family member of a counter-revolutionary or obscurantist or what have you. Stalin said in November 1937: “And we will eliminate every such enemy… we will eliminate his entire lineage, his family! Here’s to the final extermination of all enemies, both themselves and their clan.”

In our own time, Israel is practicing a form of collective punishment every time it blows up the home, often in occupied East Jerusalem, of the family of a suspected terrorist or even in some cases a teenager who got caught throwing some stones at the military. North Korea has been known to imprison the family members of dissidents.

This behavior is covered in Article 33 of the Fourth Geneva Convention, which reads in its entirety: “No protected person may be punished for an offence he or she has not personally committed. Collective penalties and likewise all measures of intimidation or of terrorism are prohibited. Pillage is prohibited. Reprisals against protected persons and their property are prohibited.”

The definition of “protected persons” is a little complicated, and you can read it here, but it includes both citizens of a given country (“nationals”) and non-nationals who find themselves in the hands of a hostile power, which President Trump’s America would surely be to Muslim non-citizens, morally if not always legally.

So now comes the question: What does Trump mean by “very tough”? He probably won’t say. Let’s grant that he doesn’t mean execution. He’s not that crazy. In his mind, he might “just” mean detaining family members, putting the screws to them, seeing what they know. Obviously, under any number of circumstances, interrogation of family members of those who commit crimes is reasonable. It happens every day, hundreds of times across the country.

But Trump sounds like he’s talking about more than that. The way he appears to think about these things, it doesn’t seem at all far-fetched to imagine that he might envision, say, detaining the family of someone who commits a future terrorist act. You know, just to teach the others a lesson.

A Trump administration could probably find some antique (or not) federal law under which to do such a thing, and then fight the inevitable court challenges and see what happens. And if such a case landed before the right kind of Federal Society judge, well, it seems unlikely that any American federal judge could possibly justify such a thing, but in a country that actually elected Donald Trump president, who knows? And would the GOP really object? After all, we already have the precedent of Republican officials from John Yoo to Dick Cheney telling us that we don’t have to bother with all that Geneva twaddle.

It’s yet another new Trump low, and it raises the specter of a lawless government ditching norms that we’ve (mostly) stood by for decades. And if we ditch them, look out, because others will too. One doubts we will, but the mere fact that the front-runner for the Republican nomination is putting this stuff into the national discourse is horrible enough. And good God, what’s coming next week?