For ISIS, the fusing of controlled territory from Deir Ezzor to Mosul is a major step towards its stated goal of creating a unified Sunni caliphate in the region.
"The conflicts in Iraq and Syria have long been fusing," says Peter Harling, the Damascus-based project director for the International Crisis Group's Middle East program. "ISIS operates across the border and steps up its activities on one side when it feels either empowered or under pressure on the other. The frontier line is eroding."
The organization, now considered more dangerous than al-Qaida by many Western officials, is run by Abu Bakr al-Baghdadi, a merchant's son regarded as the ideological heir to late al-Qaida leader Osama bin Laden.
In Syria, ISIS has made the city of Raqqa its de facto capital, taking over the city last year. But it has struggled in recent campaigns to expand the area it controls.
ISIS faltered its bid to wrest control of terrain from Jabhat al-Nusra and other rebel groups, and its attempts to implement an ultra-conservative form of religious law has been met with resistance from Syrian civilians.
Women are now forced to wear head-to-toe Islamic dress when out in public, while cigarettes and Western products like Coca-Cola have largely been banned. Earlier this year, civilians in Raqqa began to protest after the reported hangings of Syrian journalists and activists, accused of criticizing ISIS rule. Across the north and east, the group's violent tactics both on and off the battlefield have made them increasingly unpopular.
In January, ISIS emerged victorious from clashes against rebel forces led by Nusra and the Islamic Front, keeping control of Raqqa city. But the infighting forced it out of northern provinces and slowed its advance into Deir Ezzor.
"We've seen them do well in Syria before, in Idlib and Aleppo, and then there's backlash and they get kicked out," Zelin says. "They have a lot of enemies everywhere. So far, because of the general destabilization in Syria and Iraq, they've been successful. But that doesn't mean they'll have momentum."
For now, the group is claiming that it's already reaping the rewards of this week's Iraq advance.
"On Twitter, they've had pictures that they claim are American tanks from Iraq being brought into Deir Ezzor and inspected by their leaders there," Szybala says. "They claim they are already getting benefits from the spoils of Iraq."
The gains could push the Syrian army into action against ISIS. Analysts say that Iran, President Bashar al-Assad's most important regional ally, is insisting that he take action. Iran, a Shiite power, and ISIS, a Sunni insurgency, have rival goals in the power battle of Syria's war.
Szybala says there are signs the regime is beginning to take on ISIS, attacking the group's strongholds in Raqqa and Hassakah earlier this week.
"If that's really happening, I don't think it's just for show," Szybala says. I think it's probably because this is [partly] Iran's game, and now that ISIS is proving itself to have real military power and to be a trans-national threat, Iran can't afford that and would be pushing the regime to finally take action. ISIS is acting like an army now."