In the letter dated 9 October, and sent after US troops were pulled out of Syria, Mr Trump told Mr Erdogan: “Don’t be a tough guy. Don’t be a fool!”
President Trump was urging Turkey not to launch a military offensive against Kurdish-led forces in northern Syria, but Mr Erdogan ignored this request.
US Vice President Mike Pence is now in Ankara to push for a ceasefire.
View the complete October 17 article on the BBC News website here.