Shortly before Christmas, President Trump spoke with Recep Tayyip Erdoğan. The Turkish ruler promised Trump that U.S. forces were no longer needed inside Syria as Turkey would fight the Islamic State. Trump may not have realized Erdoğan’s insincerity: Erdoğan was complicit in the Islamic State’s rise and sustenance, as both intelligence and forensic evidence revealed. “Our troops are coming home!” Trump tweeted.

The sudden shift in U.S. policy led both Defense Secretary James Mattis and Special Envoy Brett McGurk to resign, but in subsequent weeks senior national security and defense officials convinced Trump to slow the withdrawal, if not change his mind completely. The reason to keep U.S. troops in eastern Syria is clear: Maintaining a small presence is a deterrent from other groups from filling the vacuum. A small U.S. contingent, for example, disrupts the so-called land bridge by which Iran’s Islamic Revolutionary Guard Corps sends fighters and supplies across Iraq and into Syria and Lebanon. A small presence is also important for intelligence gathering. Most importantly, a U.S. “trip wire” prevents other countries like Turkey from invading the area.