President Donald Trump threatened Turkish President Recep Tayyip Erdogan in a White House letter the day after he invaded northern Syria, saying Erdogan shouldn't be a "tough guy" or a "fool" as he tried pressing for negotiations to end the hostilities.

The letter was first obtained by Fox Business host Trish Regan, and it was later confirmed by The New York Times as authentic.

In the letter, dated October 9, Trump struck an aggressive tone while also holding out the prospect of a deal.

"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!" Trump said in the letter.

Erdogan was apparently unswayed by the letter, given he's since rejected calls to end military operations in northern Syria against Kurdish militias that Turkey has long treated as a national security threat.

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President Donald Trump threatened Turkish President Recep Tayyip Erdogan in a White House letter the day after he invaded northern Syria, saying Erdogan shouldn't be a "tough guy" or a "fool" as he tried pressing for negotiations to end the hostilities.

The letter was first obtained by Fox Business host Trish Regan, and it was later confirmed by The New York Times as authentic. In the letter, dated October 9, Trump struck an aggressive tone while also holding out the prospect of a deal.

"Let's work out a good deal!" the letter began. Then he threatened to destroy the Turkish economy, a remark he doubled down on soon after Turkish forces poured over the Syrian border on October 8, to wipe out a Kurdish militia that has been a critical American ally in the war against the ISIS.

"History will look upon you favorably if you get this done the right and humane way," Trump said. "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!"

Read more: Trump threatens to 'totally destroy and obliterate' Turkey's economy if it takes 'off-limits' actions in Syria

Erdogan was apparently unswayed by the letter. He has since rejected calls to end military operations in northern Syria against Kurdish militias that Turkey has long treated as a national security threat. Over 100,000 people have been displaced so far in the chaotic battle between Kurdish militias and Turkish-led forces.

Trump abruptly withdrew US forces from northern Syria early last week, effectively giving Turkey the green light to launch its military operations. The US government has since hit Turkey with sanctions over its campaign in Syria and ended negotiations on a $100 billion trade deal. He's also hiked tariff rates on Turkish steel to 50%.

Here is the text of the full letter:

Dear Mr. President:

Let's work out a good deal! 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. I've already given you a little sample with respect to Pastor Brunson.

I have worked hard to solve some of your problems. Don't let the world down. You can make a great deal. General Mazloum is willing to negotiate with you, and he is willing to make concessions that they would never have made in the past. I am confidentially enclosing a copy of his letter to me, just received.

History will look upon you favorably if you get this done the right and humane way. 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!

I will call you later.

Sincerely,

Donald J. Trump