Read: The unconstitutional strike on Syria

But whatever the political leverage offered by U.S. forces, the war itself is already decided in favor of the Assad regime. While many in the Trump administration want to avoid replicating what they see as the United States’ experience in Iraq, where a perceived early withdrawal ceded space to an extremist insurgency, the White House has offered another reason for keeping troops on the ground: Iran. Tehran has helped provide significant battlefield support to Assad’s forces, which, along with Russian help, has served to turn the tide of the conflict, and the White House’s overriding rationale for staying in Syria has shifted focus to Iran.

National-Security Adviser John Bolton stated in September that American forces would remain in Syria “as long as Iranian troops are outside Iranian borders, and that includes Iranian proxies and militias.” James Jeffrey, the U.S. envoy to Syria, said the same month that Washington was “going to be focusing on the long-range Iranian presence there and ways to get that out while we’re working on the Daesh problem,” referring to an Arabic pejorative for ISIS. And on the ground, the coalition has already used deadly force in several high-profile incidents with Iranian-backed or pro-regime forces.

It’s not a question unique to Syria either. In Yemen, the administration has responded to congressional scrutiny of its support for the Saudi-and-Emirati-led coalition by citing the danger not only of Iranian-linked Houthi rebels, but also of al-Qaeda and ISIS. But its ongoing attempt to lump justifications together—collapsing walls between what Congress has and hasn’t authorized—was dealt a blow last week, when the Senate voted 63–37 to consider a bill that would end support to the coalition.

Read: The war against ISIS will go undeclared.

The legality of any prolonged U.S. deployment rationalized by the threat of Iran is tenuous at best. When President Barack Obama ordered intervention against ISIS in 2014, he was able to justify it through the 2001 Authorization for Use of Military Force (AUMF), the salient portion of which runs just 60 words long and which was signed days after the 9/11 attacks, but which has since become the backstop of U.S. counterterror operations for 17 years.

Similarly, George W. Bush and Trump have used it as a basis for activities ranging from drone attacks in Yemen and Pakistan to air strikes in Libya and the deployment of U.S. forces across Africa. In Syria, the Obama administration used the AUMF to target ISIS—a group that didn’t exist on 9/11.

Still, no one, said Steve Vladeck, a law professor at the University of Texas whose work often focuses on the AUMF, “could argue with a straight face” that the AUMF applies to Iran, or to the Syrian government. Coalition members like the United Kingdom and France also each have their own domestic legal and political rationale for their involvement in fighting ISIS in Syria. Pulling them further into posturing against Iran may prove difficult, and they may resent the coalition being used for that purpose.