Since reaping money and military equipment in a June offensive on the Iraqi city of Mosul, the Islamic State of Iraq and Syria (ISIS) has effectively opened the border between Syria and Iraq and pushed further east through Raqqa, Deir Ezzor and Hassakeh provinces, becoming the dominant force there over Jabhat al-Nusra and other groups.

Now it has two major objectives: to overtake any remaining opposition groups and to start chipping away at areas under the Assad regime, as it began to do this month in a bloody attack on the government-held Shaar gas field in Hama province.

“Crushing hostile rebel groups thus remains ISIS’s top strategic priority in Syria, and an escalation near Aleppo that coincided with regime gains there could go a long way towards accomplishing that goal,” says Noah Bonsey, the Beirut-based senior Syria analyst at the International Crisis Group.

Here he weighs in on ISIS’s growing strength in the east, why seizing Kurdish-controlled territory became one of its priorities, and whether its reliance on foreign fighters will be supplanted by new recruits from homegrown jihadist groups.

Syria Deeply: How strong is ISIS in eastern Syria right now? As strong as we believe, or facing a greater challenge than they thought from Nusra and now the regime?

Noah Bonsey: After defeating rebel and Jabhat al-Nusra forces in Deir al-Zour earlier this month, ISIS now controls the province’s oil fields and nearly all of its population centers, with the notable exception of areas within Deir al-Zour city that remain under regime control. These gains, added to ISIS’s control of most of Raqqa province and parts of Hasakah province, make ISIS the dominant military force in non-Kurdish areas of eastern Syria.

Rebel groups and al-Nusra were ISIS’s primary foes in this part of the country. Having now defeated those forces, driving some out, while inducing others to pledge allegiance, ISIS is moving on to its next priority in eastern Syria: consolidating control within its area of dominance by attacking regime bases that remain there.

ISIS’s gains of territory and resources over the last two months are substantial, but we shouldn’t overstate the group’s power. ISIS is currently deploying significant manpower on multiple fronts in Iraq and Syria, leaving it dependent upon local partners to maintain control in much of the territory the group occupies. Given ISIS’s dismal reputation among anti-regime Syrians and its history of alienating erstwhile allies, there is reason to believe that ISIS gains in Syria could be halted and eventually reversed if the Syrian opposition’s state backers worked more seriously to improve the organization and resources of anti-ISIS rebels.

Syria Deeply: How strong is their recruitment right now? Are they attracting enough recruits from the Islamic Front and Jabhat al-Nusra so that they aren’t as dependent on foreign fighters?

Noah Bonsey: ISIS remains disproportionately dependent on foreign fighters in Syria, but recently it has won some notable defections from Jabhat al-Nusra and it appears to be attracting limited numbers of Islamic Front fighters. This should be expected, given the fluidity of the militant scene, the tendency of fighters to gravitate towards groups that enjoy better resources and battlefield momentum, and the fact that Islamic Front factions are currently experiencing resource shortages.

The big question that remains: To what extent can ISIS continue to attract recruits from groups that have waged bitter war against it in recent months? ISIS is betting that its resource advantages and the attractiveness of its “Caliphate” brand among Salafi fighters will allow it to overcome its poor reputation and the animosity generated by its abuses.

One key variable that will help determine ISIS’s success or failure in this regard is the relative viability of anti-ISIS rebel forces. Insofar as the organization, resources and performance of rebel groups improve, it will be easier for them to retain fighters and gain new recruits. On the other hand, setbacks suffered by rebel groups, whether at ISIS or regime hands, help swell ISIS ranks.

Syria Deeply: Why are the Kurdish areas so important to ISIS right now? Is it because it would give them control of a large swath of the border, or are there other factors at stake?

Bonsey: Border territory and oil are key sources of income to armed groups, and Kurdish-controlled areas in Syria are rich in both. That said, Kurdish YPG forces have proven themselves among the most effective military forces in the country, and it remains to be seen how much ISIS is willing to invest in directly confronting them.

Syria Deeply: Are you surprised that they are going after the northeast so soon after the Iraq offensive – are they stretching themselves too thin? Does it imply that their confidence is very high in Syria?

Bonsey: At the moment, ISIS is deploying significant resources towards capturing remaining regime bases in the east. In my opinion, the key question is where ISIS will seek to concentrate next once it concludes that effort. It may escalate its fights against Kurds, but alternatively it may choose to dedicate more substantial manpower towards a campaign against rebel groups, aiming to recapture ground north of Aleppo that it lost early this year. I think the latter is more likely. Regime advances in Aleppo are weakening rebel forces in the area, providing ISIS with an opportunity to exploit the situation with a renewed offensive, just as the regime did when rebels entered into war with ISIS in January.

In anticipating ISIS’s decision making, we need to keep in mind the group’s ideal scenario in Syria: the defeat of mainstream rebel groups and Jabhat al-Nusra, which would leave ISIS as the lone substantial force capable of maintaining war with the regime. This monopoly would resemble that which ISIS currently enjoys in Iraq, and would allow it to better attract recruits both inside and outside Syria. As we have seen in Iraq in the years since 2007, those who currently fight or hate ISIS may eventually grudgingly accept it, if and when they have no other potent alternative for resisting regime aggression.