The US and Turkey have agreed to carve out an "ISIS-free" zone in northern Syria, according to new reports in both the New York Times and the Washington Post. In practice, that means helping Syrian rebels take and hold a chunk of territory in northern Syria.

This is a move the administration has resisted for years, and enough remains to be worked out with this plan that it might not be implemented in practice. But if does happen, it would be a significant escalation in America's war on ISIS — and potentially the start of a much bigger role for the US in Syria.

What is the "ISIS-free" zone, and how would it work?

The idea of carving out part of Syria as a "safe zone" for Syrian rebels has been around since at least 2012. Back then, the idea was to protect civilians and train Syrian rebels to fight dictator Bashar al-Assad. Now the idea is to fight ISIS by forcing and keeping them out of a strategically important piece of territory.

The ISIS-free zone, according to the Washington Post, would be in northwestern Syria, along the Syria-Turkey border. It would extend about 68 miles along the border, and run perhaps a few dozen miles into Syrian territory.

The idea is that an as-yet undefined group of Syrian rebels, supported by US and Turkish airstrikes, would seize the territory and then keep out ISIS. Turkey recently gave the US the green light to use Incirlik, a nearby Turkish airbase, for strikes on ISIS. Turkey has been denying the US access to this base unless the Obama administration agreed to some kind of safe zone proposal, which it now apparently has.

The goal, the Times reports, is to cut off ISIS's access to crucial smuggling routes on the Turkish border and more generally protect Turkey's border from ISIS.

According to American officials, the US commitment is largely limited to airstrikes against ISIS: It will not institute a no-fly zone, nor will it pledge to attack any Assad forces that enter the area.

"What we are talking about with Turkey is cooperating to support partners on the ground in northern Syria who are countering [ISIS]," one senior Obama administration official told the Times.

Turkey appears to have a different view. It envisions this area as a protected zone for Syrian rebels and displaced Syrian civilians.

"The principle is simple. If there is any attack on civilians between Azaz and Jarablus, we will strike," one Turkish official told McClatchy. "Whoever it is, we will not allow civilians to die."

This reflects a real tension between the US and Turkey over what to do about Syria. The United States wants to focus on defeating ISIS; Turkey has, for a long time, been much more interested in defeating the Assad regime. The fact that these two countries are now embarking on a joint plan in Syria with very different end goals could be, shall we say, problematic.

What if Assad attacks the ISIS-free zone?

The problem with intervening in Syria's civil war is, and always has been, its complexity. There are so many different sides and shifting allegiances that for the US, intervening for or against anyone risks drawing the US into a broader war.

The ISIS-free zone is a good illustration of this problem. Because neither the US nor Turkey want to send their own troops into northern Syria, they rely on Syrian rebels to actually clear ISIS out of the territory. But the Syrian rebels are much more interested in fighting the Assad regime than they are in fighting ISIS. The Assad regime knows this and (correctly) sees rebels as a bigger threat to his rule than the Islamic State. Thus regime forces might be tempted to strike Syrian rebels in their safe zone.

For example, Assad's air force has spent July been dropping barrel bombs — a particularly nasty and indiscriminate weapon — on al-Bab, a town that could be within the safe zone. Will they really stop bombing al-Bab if it switches from ISIS to rebel control?

If Assad attacks rebels or civilians who are nominally under US and Turkish protection, Obama will be stuck between two awful choices. If he lets the regime kill people in territory patrolled by American and allied planes, it would humiliate the US and expose the safe zone as a lie. But if he fights back, he'd risk drawing the US into a full-blown war against Assad and the Iranian forces propping him up.

There's another dimension to all this: the Kurds

Turkey has a substantial Kurdish minority, and fought a decades-long war with the PKK, a Kurdish separatist group. Syria also has a Kurdish minority — the Syrian Kurdish rebel group, called the PYD, has been the US's most reliable ally in northern Syria. Kurdish PYD advances, backed by US airstrikes, have even threatened Raqqa, the capital of ISIS's caliphate.

From Turkey's point of view, that's a real problem. The PYD has tight links with the PKK, and the Turkish-PKK peace process that began in earnest in 2012 is in deep trouble. After the PKK killed two Turkish policemen recently, Turkish planes bombarded a PKK stronghold in Iraq.

One reason Turkey probably likes the idea of a safe zone is as a way to limit the Kurd's influence in Syria — which would mean setting back the American-backed PYD fighters.

"Most Syrian rebels say the Turkish drive for more forceful action in Syria is less about ISIS than curbing the Kurds," the Financial Times reports. "[The Kurds'] advances have allowed them to expand westward and connect Kurdish territories along the border, sparking Turkish and rebel anxieties that the Kurds could be closer to forming a state."

The al-Qaeda problem

Meanwhile, the Syrian rebels who will ostensibly be in the safe zone pose their own problems for this policy.

Alliances among Syrian rebels groups are fluid and shift often. Some of those alliances include the Syrian al-Qaeda franchise, Jabhat al-Nusra. How do you implement the ISIS-free zone without making it an al-Qaeda safe zone?

The US has had a lot of difficulty identifying rebels that are moderate enough for the Americans to be comfortable supporting. An American plan to train and equip Syrian rebels has only managed to field 60 troops. The strongest rebel force, an umbrella group called Jaish al-Fatah, counts al-Qaeda forces as some of its senior members.

Needless to say, the US couldn't support an "al-Qaeda safe zone." But given the shifting politics among Syrian rebels, it could be hard to keep them out.