ISIS is in trouble: Kurdish forces have advanced to within 35 miles of the Syrian city of Raqqa, the Islamic State's capital.

"The Islamic State is facing perhaps its most serious symbolic and meaningful threat since it declared itself a caliphate almost one year ago," the Soufan Group, a private intelligence firm focusing on terrorism, writes in a recent briefing.

Will ISIS lose Raqqa? What will happen if it does?

How the Kurds advanced so dramatically against ISIS

The push to Raqqa began on June 15. The People's Protection Units, a Syrian Kurdish militia better known as the YPG, pushed ISIS out of Tal Abyad — a strategically important Syrian town on the border with Turkey.

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."

ISIS "is barely surviving in Syria"

With Raqqa cut off from this critical supply line, YPG forces kept marching, aided by air support from the United States. By Wednesday, they had taken a town called Ain Isa, roughly 34 miles from Raqqa. ISIS, according to CNN, has begun digging in around Raqqa in anticipation of an attack.

"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," the Soufan Group's briefing group points out. "The Kurdish group is backed by coalition airstrikes and material support, and is methodically moving against the Islamic State."

ISIS is weaker than you think

ISIS, for all its victories in Syria and Iraq, is something of a paper tiger. The group's entire strategy demands that it keep expanding on all fronts, yet it lacks the material support to fend off all of its enemies. That has left it vulnerable — not at any risk of collapsing tomorrow, but weaker than it may look.

ISIS "is barely surviving in Syria," says Yasir Abbas, an associate at the private research and consulting firm Caerus Associates who closely tracks developments on the ground in Syria. "It is struggling to halt YPG advances and is out of low-hanging fruits to cultivate to feed its expanding narrative."

The whole idea of ISIS, the thing that makes it really different from al-Qaeda, is that it claims to be the actual caliphate: an Islamic State that governs and holds territory. It needs that territory to ideologically justify itself, to collect the extortionary "taxes" it uses to fund its activities, and to attract ever more foreign recruits.

"When they declared the caliphate, their legitimacy came to rest on the continuing viability of their state," Daveed Gartenstein-Ross, a senior fellow at the Foundation for the Defense of Democracies, told me in October.

ISIS simply does not have the forces necessary to hold all of that land. The CIA estimates that ISIS has between 20,000 and 31,500 fighters; some private sector sources suggest that figure may be closer to 100,000.

ISIS's enemies, meanwhile, number far greater. There are about 48,000 official Iraqi government soldiers, but they're joined by 75,000 to 120,000 Shia militiamen fighting on the government's side. The BBC reports that there are 190,000 Kurdish peshmerga in Iraq's north. And that's just Iraq.

"Airstrikes killed thousands of ISIS troops"

In Syria, ISIS has gotten relatively lucky: The Assad regime and the various anti-government factions have been too busy fighting each other to devote sufficient effort to sweeping out ISIS. But the YPG — an organized, experienced fighting force that has overcome ISIS before, in the Kurdish city of Kobane — is a different matter.

This YPG offensive comes at a time when Iraqi forces are, somewhat haphazardly, attacking ISIS strongholds in western Iraq, splitting the group's attentions. And US airstrikes, limited as they are, are bleeding the group.

ISIS's defeats near Raqqa are the result of "a combination of overstretch and increased YPG capacity due [to] the air support they are getting from the US," Abbas said.

"Airstrikes killed thousands of ISIS troops," he elaborated, "and even though ISIS has been able to replace some of them with new recruits, it can't replace its experienced fighters that it relies a lot on." That's making it very hard to fight the YPG and the Iraqis at the same time.

"Their mobility is high," Abbas says, "but not high enough to meet all of these challenges."

Why Raqqa matters

All 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: The group is composed of smart fighters who can react rapidly to serious crises. They've already mounted a serious attack against the Syrian government in al-Hasakah, and a less significant attack on Kurds in Kobane.

These counterattacks allow ISIS to "apply pressure on soft targets and relieve pressure on Raqqa," Abbas says. Kurdish fighters may not want to stray too far from Kurdish territory toward Raqqa at a time when it's under attack.

"Taking on Raqqa City is a major endeavor ... that I doubt the YPG are willing to [embark on] at this time," Abbas suggested. But, he added, "I would not completely rule out the possibility of YPG and their affiliated groups attacking Raqqa City. ... US support to the YPG and [ISIS's] military engagements in Iraq and Syria may change YPG calculations."

It's not clear if the US would, or even should, give the YPG the amount of support necessary for such an undertaking: There are reports of YPG fighters ethnically cleansing Arabs from the territory they take.

Even a serious attack on Raqqa proper would be a real blow to ISIS. Until now, the group's control over Raqqa has been virtually unquestioned, symbolizing ISIS's ability to hold and govern territory. If Raqqa were to come under siege, it would call that into doubt in a very public way.

"Raqqa is not likely to fall," the Soufan report concludes. "It will likely become a city under siege, however, and be more or less surrounded. That alone would be a big blow to the group; the capital of what it sees as an ever-expanding caliphate so obviously contained."

This would hardly be the end of ISIS. But it would be a major victory in a theater that, for a long time, seemed utterly devoid of hope.