A letter by President Donald Trump urging the Turkish leader Recep Tayyip Erdogan not to "be a tough guy" over his bombardments in northern Syria, breaches all standards of diplomacy and etiquette and shows that the U.S. president is mentally unwell, a political historian has said.

...

News networks expressed bewilderment at the letter, whose date suggests that it was ignored by Erdogan, who proceeded with his attacks on northern Syria.

Among those on Twitter voicing their bemusement, was U.S. foreign relations expert Brad Simpson, an associate professor at the University of Connecticut.

In a thread he tweeted to his 14,000 followers, Simpson said: "I am a historian of U.S. foreign policy. I have read many, many letters from U.S. Presidents to foreign leaders, and I have never read a letter from the U.S. President so unhinged, so threatening, so bizarre, so completely lacking in basic etiquette. Trump is deeply, deeply unwell."

Simpson concluded that Trump had dictated the letter straight to a secretary without any input from top state department, or Pentagon advisers and that, "it's hard to overstate how crazy this is, given the stakes involved for U.S. in Turkey's invasion of Syria."