The arrival of ISIS fighters in Syria's capital raises the stakes for the broader region, directly threatening Syria's government and bringing the terror group closer to some of the region's last stable countries.
Fighting in and around the Yarmouk refugee area in southern Damascus puts thousands of civilians in serious danger, prompting the United Nations to call Tuesday for an evacuation of the area. And it establishes a new ISIS foothold that's much closer to Jordan, Lebanon and Israel.
"I think this is shocking to a lot of folks," said Brian Katulis, senior fellow at the Center for American Progress.
Yarmouk has been held for three years by more moderate factions opposed to Syrian President Bashar al-Assad. But fighting escalated severely starting last week, when ISIS made a push to take over the area. All this has made the violence in Damascus even more fractious than before, involving the Syrian military, various rebel groups (including at least one linked to al-Qaeda) and ISIS.
"This is very surprising because of its proximity to the heart of the regime," Katulis said. "It's actually one of the first signs of a major outpost of ISIS in a place outside of the northeastern part of the country."
It brings ISIS to within about 60 miles of Jordan's northern border, fewer than 40 miles from the Israeli-controlled Golan Heights, and about 20 miles from Lebanon. Jordan is notable among Arab states by being Western-friendly and having good relations with Israel. It's also a outpost of stability as fighting rages in Iraq, Syria, Egypt, Libya and now Yemen.
The current fighting won't disrupt supplies of Middle Eastern oil—there are no major wells or pipelines near Damascus. But like many recent events in the region, it contributes to a general feeling of anxiety among investors.
"I think it just adds to this overall sense that things continue to be perhaps more unstable than we'd like. It's more something that affects market psychology than disrupting barrels," said Guy Caruso, senior adviser at the Center for Strategic and International studies and a former head of the Energy Information Administration under President George W. Bush.
ISIS is trying to both sweep other opposition groups from the area and to establish a foothold in Damascus, close to Syria's seat of power, said Blaise Misztal, director of foreign policy at the Washington-based Bipartisan Policy Center.
"It's trying to prepare the battlefield for an eventual attack on Assad's power base," he said.
No United Nations food, water or medical supplies have reached Yarmouk in a week, creating a situation on the ground that's "completely dire," NBC News reported Tuesday.