Last month, video of a Syrian rebel apparently beheading a child captive stunned the world. In the amateur cell phone footage, a fighter from Aleppo faction Harakat Nour al-Din al-Zinki (also known as Zinki or al-Zinki) sawed off his prisoner’s head in the back of a pickup truck.

The video reverberated worldwide and invited near-universal condemnation, including from the Syrian opposition itself. “This horrific video showing the beheading of a boy suggests some members of armed groups have truly plumbed the depths of depravity,” said Philip Luther, Director of Amnesty International’s Middle East and North Africa Program. Zinki itself condemned the act and vowed to hold the perpetrators accountable.

The episode and its aftermath ought to prompt the United States to consider what exactly it’s trying to achieve with its Syrian proxy war, and what it’s willing to tolerate to achieve it. Because, in practice, proxy warfare sometimes involves atrocities in truck beds, in addition to many other inhuman acts of war that aren’t filmed or posted on Facebook.

If the United States cannot absorb its proxies’ bad behavior, on and off camera, then its strategy in Syria is likely untenable. If, on the other hand, the United States really is serious about pursuing a two-pronged strategy—challenging the Syrian regime of Bashar al-Assad militarily while backing rebel allies to counterbalance jihadists within the Syrian opposition—well, then it will have to back Zinki and other groups like it.

Contrary to some reporting, Zinki was not “U.S.-backed” at the time of the beheading. Zinki’s support from the Müşterek Operasyon Merkezi (MOM)—a joint operations room in Turkey that reportedly includes the CIA and allied intelligence services—was actually cut off in August or September of last year, rebel sources told me.

But Zinki had recently been negotiating with state backers, including the United States, over the restoration of its support. Representatives from Zinki told me a meeting with these states—now postponed—was scheduled on the evening of the beheading.

The United States has so far avoided a categorical denunciation of the group. The U.S. line, at least in public, seems to be that this is a specific episode that is being investigated.

“We routinely vet the groups we work with and support, as you might expect, and their human rights record figures prominently in that,” a U.S. official told me on condition of anonymity. “We do not support groups that condone this sort of barbarity, period. We encourage al-Zinki to investigate the incident and expect all parties to comply with their obligations under the law of armed conflict.” The official declined to comment on which specific factions are backed by the United States.

The reasons why Harakat Nour al-Din al-Zinki is effective are frequently the same reasons why it is a problematic, difficult to control proxy.

Harakat Nour al-Din al-Zinki has a reputation for being organized, militarily effective, and, unlike some other rebel brigades in northern Syria, a genuinely autonomous rival to jihadists. It has skirmished with former Syrian Al Qaeda affiliate the Nusra Front (now the Fateh al-Sham Front), although lately the two have fought side-by-side outside Aleppo.

Zinki is one of the rare rebel factions with a proven track record of fighting effectively outside its home area, and it played a major part in expelling the self-proclaimed Islamic State from the Aleppo area. The brigade is an uncommon example of comparative pluralism, including Arabs, Turkmen, and Kurds, as well as both religious and relatively secular members. Although some analysts label Zinki “Islamist,” the brigade’s animating philosophy seems more like “order.”

“We didn’t rise up for chaos—we want the rule of law”

“We didn’t rise up for chaos—we want the rule of law,” Muhammad al-Sayyid, head of Zinki’s political office, told me in a meeting in the Turkish border town of Reyhanli. Sayyid described the group as a “local resistance movement.”

According to development workers, diplomats, and other rebels with whom I’ve spoken, Zinki has created a unified system of top-to-bottom control in its home areas in the western Aleppo countryside, where it has kept civilian life functional and kept jihadists out. Of course, this order comes with a price—Syrian activists and international rights groups have documented instances of arbitrary detention and torture in areas under Zinki control.

When Zinki’s external support from the U.S.-led operations room was cut off, the rebel group was able to sustain itself with its own diversified economic base, including revenue from factories it controls.

Instead of disintegrating, a Zinki without foreign backing continued to absorb new fighters and factions—something the Zinki representatives with whom I spoke said may have gotten the group in trouble.

The fighter who beheaded the captive, they told me, had joined recently amid the desperate, chaotic battle to stop the regime from besieging Aleppo city’s rebel-held eastern neighborhoods.

The real age of the executed captive is disputed—some contend he was actually an adult combatant—but everyone concedes that he was beheaded by a Zinki fighter. Sayyid and another Zinki representative, Yasser al-Yousef, with whom I spoke over WhatsApp, said the captive was given medical treatment and was being taken to a prison when a fighter whose brother had been killed the day before got out of control.

The other fighters with him “thought he was engaging in some sort of ‘psychological warfare’ against this soldier,” Yousef told me. “No one thought this sort of talk would move to something serious.” By the time he moved to actually behead the captive, said Yousef, the others didn’t think they could stop him without sparking a shootout.

Videos from before the execution and the hair-raising execution video itself, available on social media, don’t necessarily support Zinki’s account. In footage from before the beheading, Zinki fighters including both the perpetrator and local Zinki commanders taunt the captive and joke about beheading him. Local Zinki commander Omar Salkhou poses for a picture and says, “Selfie with the captive behind me.” (It rhymes in Arabic.) In the video of the beheading, others out of frame can be heard shouting, “God is great.”

Zinki swiftly issued a statement condemning the beheading, saying it had detained those responsible and would form a special tribunal to try the perpetrators.

“Some in revolutionary circles have tried to downplay this crime, to say, ‘It’s a reaction that might be justified, he lost his brother,’ and so on,” Yousef told me. “But whatever the justifications were, we won’t accept them.”

Sayyid met with the head of the opposition National Coalition and invited the Coalition and the opposition Interim Government to consult on the legal framework for the tribunal’s work.

Yet this tribunal has been delayed (although Sayyid and Yousef said it was still moving forward). Until rebels broke the siege of east Aleppo earlier this month, the perpetrator, now detained, was trapped inside rebel-held Aleppo. The route into the rebel half of the city reportedly remains dangerous and only intermittently passable.

And some independent judges invited to supervise the tribunal have refused to participate, saying the participation of courts that operate in Zinki areas under the group’s protection makes a fair trial is impossible.

The other Zinki fighters who appeared in the pre-beheading footage of the captive have already been questioned and released, Yousef confirmed to me. Local commander Salkhou apparently led Zinki’s force inside Aleppo city in its push to break out of the siege. He was prominently featured in Zinki’s media releases celebrating the victory.

Zinki’s narrative that it inadvertently took in bad actors echoes the main reason its support was cut off in the first place, after it absorbed a faction that included a commander who had conspired with the Nusra Front to kidnap and ransom Italian relief workers.

At this point, it seems unclear whether Zinki’s half-steps towards accountability will be enough for its would-be foreign backers.

Sayyid and Yousef both stressed Zinki’s commitment to a fair and independent trial, but organizing a perfectly credible trial anywhere inside Syria is, realistically, probably impossible.

But state backers did not want to restore support to Zinki because of its beatific membership. They were looking hard at Zinki because the group is strong and, in and around Aleppo, a credible counterbalance to jihadists. That Zinki was blacklisted by the MOM and seems to have grown only more powerful was a signal that the group was a force to be reckoned with.

Zinki’s representatives told me they had been in touch with U.S. and other foreign diplomats in an attempt to resolve the crisis.

“It’s unfair to assess [Zinki], which is considered among the biggest revolutionary forces—militarily and politically—in north Syria on the basis of one episode,” said Yousef.

And Zinki is not the only party in Syria that has crossed lines, or even—by a long measure—one of the worst.

Over five years of war, people have done all sorts of wild, monstrous things in Syria. Chief among them is the Assad regime, which regularly eviscerates families with its indiscriminate aerial bombing, but other parties on all sides have been guilty of violations as well.

While the Zinki beheading is a singular, extreme example, it helps clarify the choice the United States and its allies currently face in Syria.

If the United States wants to reinforce a meaningful non-jihadist constituency within Syria’s armed opposition and retain its own influence by proxy, then Zinki is a natural, if unpalatable, partner. But if Washington insists on keeping its hands perfectly clean, there’s probably no Syrian faction—in the opposition, or on any side of the war—that merits support.

Sayyid put it to me in blunt terms: “If they try to weaken Zinki, Al Qaeda will get stronger.”

This is where Syria’s war is right now, and it’s the choice with which Washington has been presented. Washington can exclusively back rebel factions that are ostensibly clean but that have limited efficacy or are actually under the thumb of the Fateh al-Sham Front or factions like Ahrar al-Sham. Or it can seriously invest in Syria’s rebels as they currently exist, including Zinki, in ways that might shape the battlefield and the character of the armed opposition.

A proxy strategy that does anything less—one that can’t stomach Nour al-Din al-Zinki—is just playing around.