Turkey has gathered forces on its southern border with Syria, as it prepares to make good on its plans to carve out a “safe zone” by pushing out Kurdish fighters who have been United States allies but which Ankara considers terrorists.

As President Trump pivoted from defending his sudden decision to withdraw U.S. troops from the region to warning Turkey against “any unforced or unnecessary fighting,” the Kurdish-led civilian administration in northeastern Syria mobilized for what appears to be an imminent invasion.