If the objective is to remove the ISIS as a functioning guerrilla army, then an alliance with Syria would make perfect sense. Might even keep trying to make peace with the Iranian Islamic Republic. But for sure, engage the Kurdish Peshmerga, give them weapons in full measure, and tell them they can keep all the territory they capture.
Kurdistan is the best buffer state to set up among all those Sunni Islamic jihadists, and even the Shi’ite Islamic jihadists would end up respecting them.