U.S. President Donald Trump warned his 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 north-eastern Syria, The Guardian reports.

The letter, first obtained by a Fox Business reporter, was shorn of diplomatic niceties and 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.

Days after appearing to greenlight an invasion by pulling U.S. troops from the Kurdish-dominated region, Trump told the Turkish president he would wreck Ankara’s economy if the invasion went too far.

“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”.

On Wednesday, October 16, Trump also hailed his own decision to withdraw US troops from Syria as strategically brilliant and declared the Kurds were “much safer now”, contradicting the official assessment of both the state and defense departments that the Turkish offensive was a disaster for regional stability and the fight against Isis.