In the US-backed Iraqi invasion of Mosul, Iraq not only used its own military, and the Kurdish Peshmerga, but Shi’ite militias as well. With the fight over Mosul limited to the Old City now, the militias are moving eastward, toward the Syrian border.
With some Shi’ite militias already fighting alongside the Assad government in Syria, the possibility of more crossing in from Iraq to confront ISIS forces on the eastern front could be very appealing for the Assad forces, pushing ISIS away from the border and into a smaller pocket of territory.
It might be a difficult problem for the US, however, as they’ve long opposed Assad, and condemned Shi’ite militias backing him in Syria as “Iran-dominated.” The militias have been tolerated within Iraq, since the US supports the Abadi government, but they could quickly find themselves at odds with the US, in crossing borders even without changing their goal of fighting ISIS.
It also likely raises questions for Iraq, for while they’re more supportive of Assad in Syria, they have also gone to great effort to bring the militias under government control, and in seeing militias crossing into Syria, they are effectively moving outside of Baghdad’s direct control.