US president Donald Trump announced by tweet on December 19 his intention to withdraw US troops from Syria. This followed a phone call between Trump and Turkish President Recep Tayyip Erdogan, who had often stated his intention to invade north-eastern Syria.
Trump’s announcement was widely seen as giving a green light for the planned Turkish invasion. Trump’s decision caused dissension within the US government. Defence secretary James Mattis resigned, as did some other prominent officials.