US President Donald Trump warned his counterpart Turkish counterpart, Recep Tayyip Erdoğan, “don’t be a fool,” and said history risked branding him a “devil” in an extraordinary letter sent the day Turkey launched its incursion into northeastern Syria, The Guardian reported.

The letter, first obtained by a Fox Business reporter, was shorn of diplomatic niceties, and it began with an outright threat.

“Let’s work out a good deal!” Trump wrote in the letter dated October 9, whose authenticity was confirmed to various news outlets by the White House.

“You don’t want to be responsible for slaughtering thousands of people, and I don’t want to be responsible for destroying the Turkish economy – and I will,” he wrote.

“History will look upon you favorably if you get this done the right and humane way,” Trump continued. “It will look upon you forever as the devil if good things don’t happen.”

“Don’t be a tough guy. Don’t be a fool!” he finished, adding: “I will call you later.”

The bizarre letter was met with incredulity, with many at first questioning its legitimacy and some calling it a “joke” and an “embarrassment.”