Open fighting has broken out between two Syrian rebel groups, one moderate and one extremist, in the northwestern town of Azaz, along the Turkish border. Control of any border crossing is significant, but more importantly, this marks what Reuters calls the worst fighting yet between the two prominent rebel groups.

If rebel in-fighting expands beyond this one part of the country – and there's good reason to think it eventually will – it could mark a major shift for Syria's two-year civil war, one with potentially disastrous implications for the country.

Here's what happened in Azaz: The town was controlled by a rebel group called the Free Syrian Army (FSA), a loose coalition of generally moderate rebels that's been around for much of the war. On Wednesday, the FSA forces there were attacked and ousted by members of a very nasty al-Qaeda linked group called the Islamist State of Iraq and the Levant (abbreviated ISIS because they use "al-Sham" for Levant).

Tension between local FSA and ISIS forces had been getting quite serious. On Monday, an ISIS commander told Time's Aryn Baker that they were fighting war profiteers in "the so-called FSA" and working against corruption. "We launched the campaign to purify the revolution," he said.

Violence between the FSA and ISIS, two of Syria's most prominent rebel groups, has been "a long time coming," as Richard Hall of The Independent put it. Way back in April, in a different part of Syria, The Washington Post's Liz Sly reported on divisions between rebel groups so severe, and widening so rapidly, that she found many Syrians "bracing for what they fear will be another war, between the relatively moderate fighters who first took up arms against the government and the Islamist extremists who emerged more recently with the muscle and firepower to drive the rebel advance." An FSA commander told Sly that a fight between the rebel groups was "unavoidable." He added, "If it doesn’t happen today, it will happen tomorrow."

We're not at the point of a violent, nationwide split between the FSA and ISIS yet, but this is a pretty strong sign that it's coming. This is bad news for Syria – potentially very bad. There's also the slightest touch of a possible silver lining as well, but even that could just make things worse.

ISIS appears to be a far more effective fighting organization than the FSA. They're also, by all reports, pretty awful for the civilians living under their rule. Open tension between the two, and a willingness by ISIS to displace the FSA, means more Syrians can expect the group's severe and often violent ultra-Islamist rule.

Rebel in-fighting could also indirectly strengthen regime forces loyal to Syrian leader Bashar al-Assad, who can sit back and watch as his two biggest enemies turn against one another. Assad already appears to have a slight edge in the war; this makes it more likely that he'll believe he can fight his way to victory, no matter how long it takes or how much of the country he destroys in the process, rather than accepting a negotiated peace deal as the United States advocates.

It gets worse. Even if Assad eventually leaves voluntarily or is toppled outright, rebel in-fighting sets up the country for a second war, as Sly discussed in her April story. There's nothing like a power vacuum to make those sorts of divisions even worse.

Some analysts might be tempted to see a silver lining here. The United States has long been wary of giving Syrian rebels much support because it doesn't want to aid the al-Qaeda-allied ISIS, and it's really tough to aid one rebel group without indirectly helping another. If the FSA and ISIS divorce, that would theoretically make it much easier for the United States and other Western countries to back the FSA without worrying about indirectly helping al-Qaeda. Indeed, as the FSA inevitably loses ground to ISIS, it could be an imperative. If you think that a stronger FSA is in Syria's interests, either because you want to see the rebels win outright or just to balance the battlefield enough to convince Assad that he can't win and should cut a negotiated peace deal, then it's potentially good news that the United States might feel freer to give the FSA a boost.

It wouldn't be Syria unless even the good news were also bad news, though. The United States has seen this movie before, in Afghanistan. During the 1980s, the United States backed its favored Afghan rebel groups to oust the Soviet military occupation. After the Soviets left, those rebel groups fought among one another, a second war that proved even worse than the first. Because chaos and militancy tend to breed extremism, the group that emerged from Afghanistan's chaos was the Taliban. They took over and sheltered al-Qaeda, leading to the Afghanistan's third war in a row, when the United States invaded in 2001.

That doesn't mean that a post-Assad Syria will necessarily look like 1990s Afghanistan. But it's a real possibility. And deepened rebel infighting, whether one side is backed by the United States or not, risks making that more likely. Yes, it also opens the possibility of a Western-backed FSA emerging as Syria's savior, but it's a long shot.