This recently changed for a number of reasons, the first being the geography of the war. When ISIS held almost one-third of Syria, U.S. proxies could fight the Islamic State and still keep their distance from the regime, whose main strongholds lay on the other side of the country. Now, however, as potentially mutually hostile forces capture ISIS territory from different directions, they are operating in closer proximity. Second, until Russia's 2015 intervention began to yield results, the Syrian regime and the Iranian proxies allied with it were largely preoccupied with containing the insurgency in western Syria and securing Assad’s survival. Now that the rebellion is apparently under control—and particularly since the fall of Aleppo, the rebels’ last major urban stronghold, at the end of 2016—resources have been freed for use against ISIS (and U.S.-backed forces capturing the group’s territory). Finally, the Trump administration appears more willing to risk escalation against Iran in Syria in pursuit of its objectives, or at the very least is deferring to the military on whether or not to do so.
But the deeper reason for the growing confrontation is the nature of the war itself. In launching strikes on ISIS in Syria, the Obama administration wanted to avoid entanglement in the Syrian Civil War and the intrinsic risk of escalation with Iran. In reality this was never a tenable strategy, because Assad and Iran (rightly) never saw these as separate wars. The struggle for Syria is one over its territory, resources, and identity, and ISIS is just another contestant. Whatever territory ISIS loses must go to a player in a zero-sum competition among all Syria’s other warring factions.
And some places are greater prizes than others. As both regime and U.S.-backed forces converge on the ISIS-dominated Euphrates River Valley in eastern Syria, strategic resources such as oil reserves, arable land, water, and the Syrian-Iraqi border are at stake. Neither Assad nor Iran can afford to allow these to fall to U.S.-backed forces—that would pose a permanent threat to the regime, deprive it of vital resources, and threaten Iran’s strategic depth and its supply line to Hezbollah in Lebanon. Given this, Assad and Iran will either try to reach and take these areas before the United States and its allies do, or target them after U.S. interest wanes following the defeat of ISIS.
This is where the American attempt to compartmentalize what are really two dimensions of the same war collapses. The United States has in fact always been a party to the Syrian Civil War, simply by fighting ISIS. U.S. efforts have at times freed up regime resources that would otherwise go to fighting ISIS for use against other insurgent groups. On the other side, U.S. efforts have also allowed its local Kurdish and Arab partners to capture territory and resources crucial to the regime and Iran. It is no longer possible to avoid the choices this strategy presents: Cede ISIS-held territory to the regime and Iran sooner or later, including areas currently held by U.S. partners should they come under attack; or meet Iranian and regime challenges through deterrence, violence, or long-term investment in local partners that can stand their ground indefinitely. It is unclear that the U.S. administration has deliberately chosen a policy, but events on the ground have a momentum of their own.