A small band of US special forces is heading to Syria, and many observers believe they have an ambitious goal: help topple ISIS in its de facto capital city, Raqqa.

While their objective hasn't been publicly announced, media reports suggest they'll be going to work with the Kurdish and Arab forces pressing the area around the city — which is widely seen among experts and reporters as a sign that they're going to help with a push toward Raqqa.

The ISIS capital became vulnerable after a major ISIS defeat earlier this year. Kurdish forces, taking advantage of the fact that ISIS is overstretched and fighting on a huge number of fronts, cut off a critical supply line in northern Syria. Since then, they've advanced toward ISIS's nearby capital — and now American special forces are coming in to help them finish the job.

If ISIS is defeated in Raqqa, it'd be a huge blow to the group. But the operation won't be easy. And it comes with some very serious risks.

How American allies got so close to ISIS's capital

The push to Raqqa began this summer, on June 15, when Kurdish militia fighters called the PYD overran a key supply town called Tal Abyad. This seriously crimped ISIS's ability to supply itself in northern Syria — including Raqqa.

At the time of Tal Abyad's fall, the Washington Post's Liz Sly called it "a stunning defeat, [ISIS's] first major reversal since it was driven out of the Iraqi city of Tikrit in April, and one that could prove far more consequential."

As Sly explained, "Tal Abyad commands the major trade and smuggling routes on which the Islamic State has relied for its supplies from the outside world and, most significant, the flow of foreign fighters to Raqqa, the first major city it conquered."

With Raqqa cut off from this critical supply line, YPG forces kept marching, aided by air support from the United States. By the end of June, they had taken a town called Ain Isa, roughly 34 miles from Raqqa. Today they're roughly 30 miles away, according to Secretary Carter. Kurdish forces in Iraq have cut ISIS supply lines running to the country, setting the stage for a serious offensive.

To give you a sense of how big a deal it is, take a look at this map of territorial control in Syria from May from the Institute of the Study of War. You'll notice in the north, near Raqqa (labeled Ar-Raqqah on the map), a lot of ISIS gray-black:

Now take a look at the September map. A lot of that gray-black has receded, replaced by YPG purple:

"What makes the threat to Raqqa so serious is that the YPG is not an ad hoc group of rebel fighters looking to make an easy score and then rest," a June briefing from the Soufan Group explained. "The Kurdish group is backed by coalition airstrikes and material support, and is methodically moving against the Islamic State."

The US is trying to build on this progress

The US has recognized that the Kurds are its most capable military partners in Syria, and that Raqqa is their most promising front. US strategy in Syria, then, is reacting to these developments on the ground — trying to help the Kurds and allied Arabs push south into Raqqa proper.

In his Tuesday testimony, Secretary Carter labeled Raqqa one of the "three R's" that would be central to America's new anti-ISIS strategy (the other two are the Iraqi city of Ramadi and raids on ISIS territory).

"The [anti-ISIS] Syrian Arab Coalition, which we hope to strengthen through our new equipping approach... will work over time with other Syrian anti-ISIL forces to push toward Raqqa," Carter said on Tuesday, using a US government acronym for ISIS. He continued with a line that's crucial to understanding the new deployment:

While the old approach was to train and equip completely new forces outside of Syria before sending them into the fight, the new approach is to work with vetted leaders of groups that are already fighting ISIL, and provide equipment and some training to them and support their operations with airpower.

The new special forces deployment makes a ton of sense in light of that new mission. Per Reuters, the US will only be sending about 20 to 30 special operators as part of an "advise-and-assist" mission: not enough to make a difference as combat troops, but enough to potentially make YPG and Arab proxies more effective through training and coordination.

The focus will be "strategy, strike targeting & Arab-Kurd coordination," Charles Lister, a Syria expert at Brookings Doha, writes. This will be "very small & advisory only — absolutely no desire for casualties/losses to ISIS." So they'll be calling in calling in airstrikes and helping set strategy, both of which could be quite helpful to YPG forces.

The fall of Raqqa would be a huge deal — but the focus on the city could be risky

That said, don't expect YPG troops to seize Raqqa's city center in the near future. ISIS is in deep strategic trouble, but it's still tactically capable: It has a lot of smart fighters who can react rapidly to serious crises.

"This is not going to be a dash to Raqqa," Michael Knights, the Lafer fellow at the Washington Center for Near East Policy, said on Tuesday. "This is going to be an isolation campaign."

If Raqqa were to fall, it would be a devastating blow to ISIS's claim to control and govern an actual state. Hell, even a serious attack on the city would hurt ISIS. Until the past few months, the group's control over Raqqa had been virtually unquestioned, symbolizing ISIS's ability to actually govern territory. If Raqqa were to come under siege, it would call that into doubt in a very public way.

Raqqa is "obviously central to Daesh's holdings," Noah Bonsey, a senior Syria analyst at the International Crisis Group, told me on Tuesday (using an Arabic term for ISIS). "It's operated as something of a de facto capital, it's central to the Daesh narrative, and there's also resources nearby."

But there are serious risks to this focus on Raqqa.

The city's population is mostly Arab, and the US allies there are mostly Kurds. In his testimony, Carter has mentioned a group called the Syrian Arab Coalition as a US partner, but according to Bonsey, "that's a phrase that one hears US officials use; it's not a coalition that's well known or understood in conversations with Syrians themselves."

Kurdish forces may have little interest in risking a major assault to liberate non-Kurdish territory. And even if they do, wielding Kurds against Arabs could drive locals into ISIS's arms. "The prospects for success are far from clear," Bonsey said, "and the potential for Daesh to benefit for recruitment even as it loses some territory is real."