Zur Begründung sagte Außenminister Rex Tillerson am 19.1.2018:

American troops will remain in Syria long after their fight against the Islamic State to ensure that neither Iran nor President Bashar al-Assad of Syria take over areas that have been newly liberated with help from the United States

„The American Conservative“ kommentiert das am selben Tag so:

„The U.S. has no business in having a military presence in another country without its government’s permission

and it has no right to maintain that presence for the explicit purpose of preventing that government from exercising control inside its own internationally recognized borders. If another state did what the U.S. is now doing in Syria, Washington would condemn it as an egregious violation of international law and would probably impose sanctions on the government in question.“  - more: