LONDON — The battlefield successes of ISIS in Iraq, and renewed American military action there, have turned attention back to Syria. It was there that ISIS originally ramped up its appeal while fighting against the Syrian government. Today, ISIS is headquartered in Syria and uses Syrian territory to regroup and resupply.

In Western capitals there is now a renewed debate about how to deal with Syria’s brutal ruler, Bashar al-Assad. The policy options being discussed have largely been boiled down to a binary choice: jump into bed with Mr. Assad to defeat ISIS, or double down on the halfhearted existing policy of building a strong Sunni opposition.

But exclusive reliance on either of these tracks will likely fail. ISIS’ advances in Syria can’t be contained without the force that is most able to challenge it: Mr. Assad’s military. And an approach that lacks sufficient support from alienated Sunnis won’t hold back the ISIS tide over the long term.

Western policy needs to move beyond this false dichotomy.

In Iraq there is a clear understanding that military progress requires an alliance with the Shiite-dominated army and government, as well as with allies in the Kurdistan regional government. There’s no appetite for repeating the mistakes of 2003 and undertaking a full military reoccupation; any footprint today will be light — and everyone recognizes that political inclusion and empowerment of Sunnis will be crucial this time.