ISIS holds a huge amount of territory in Syria

The crisis in Syria has been one of the most important reasons ISIS has grown to be so strong — and that’s part of why the US started bombing ISIS positions there.

The ongoing civil war in Syria played a key role in ISIS’s revival, allowing it to hold on to territory and to build up weaponry and money. ”The war gave them a lot of access to heavy weaponry,” Michael Knights said. ISIS also “has a funding stream available to them because of local businesses and the oil and gas sector.”

It was also hugely important as a safe zone for the group. When ISIS was being pressed by Iraqi forces and Syrian Kurds, being able to shift supplies to different fronts and hide in safer parts of both countries is crucial. In Raqqa, its Syrian capital, and other Syrian holdings, ISIS actually governs according to hard-line Islamic law, helping support its claim to be the legitimate Islamic caliphate — a key part of its recruiting pitch and internal religious ideology.

The American-led bombing campaign is, in theory, designed to address this problem. In practice, however, it’ll be harder to drive ISIS out of Syria than out of Iraq.

In Iraq, US planes have worked closely with local forces to push back ISIS. In Syria, by contrast, the US has no sufficiently powerful ally on the ground that can clear and hold territory in much of the country (the Kurdish allies are wary of going into majority-Arab territory held by ISIS).

The war between Syrian leader Bashar al-Assad’s government and the different rebel groups has allowed ISIS to operate with relative impunity. Neither Assad nor most rebel groups see ISIS as one another’s primary threat, so they focus on other enemies. That includes the moderate rebels that the US would like to support.