A major battle to liberate the Islamic State group’s stronghold of Raqqa in northern Syria is looming, with U.S. officials looking to build on momentum from victories on the battlefields of Mosul in Iraq. The Pentagon has drawn up a secret plan to do that, likely leaning on local allies with stepped-up American support.

The question is: who are those local allies in the tangled mess that is Syria’s conflict?

Syrian government forces, Turkish troops and their Syrian militia allies, and U.S.-backed Kurdish forces all have their eye on Raqqa. Each vehemently rejects letting the others capture the city and would likely react in anger should the United States support the others. And it is not clear that any has the resources to take the city on its own. […]