President Trump sent Turkish leader Recep Tayyip Erdogan an ominous letter last week after their Oct. 6 call, after which Trump controversially announced that the U.S. would withdraw all troops from northeast Syria ahead of a Turkish military operation in the region.

The Oct. 9 letter, obtained by Fox Business Network's Trish Regan on Wednesday after Trump referenced it during a press conference the same day, advised Erdogan not to be a "tough guy" or a "fool" when it comes to his country's fragile situation at the Syrian border.

Erdogan on Oct. 9 launched a Turkish military offensive into Syria, claiming it wanted to "neutralize terror threats" and establish a "safe zone." Turkish forces last Wednesday carried out airstrikes and later announced ground troops had invaded northeastern Syria. Nearly all U.S. troops there have been removed and will be redeployed in the region in the coming weeks.

The president has been criticized by some analysts and political observers who insist he gave Turkey the "green light" to invade Syria and fight the Kurds, U.S. allies in recent efforts to bring the Islamic State to heel. Trump has defended his actions to "bring our soldiers back home" and put a stop to the "crazy endless wars," saying the United States is "7,000 miles away."

"I didn't give them a green light," Trump said Wednesday, "and if anybody saw the letter, which can be released very easily if you'd like, I can certainly release it -- but I wrote a letter right after that conversation, a very powerful letter, that was never giving a green light."

Turkey's military offensive "didn't surprise me at all," Trump told reporters. "They've been warring for many years -- it's natural for them, they fight."

But "why are we protecting Syria's land?" Trump asked, adding that the war-torn country "doesn't want Turkey to take its land. But what does that have to do with the United States of America?"

TURKEY HOLDING 50 US NUCLEAR BOMBS 'HOSTAGE' AT AIR BASE, REPORT SAYS

In his letter to Erdogan, Trump said, "Let's work out a good deal!"

History "will look upon you forever as the devil if good things don't happen," the president wrote. "Don't be a tough guy. Don't be a fool!"

"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," Trump said. "I've already given you a little sample with respect to Pastor Brunson."

Pastor Andrew Brunson was released one year ago from Turkey, where he was held as a political prisoner for more than two years. The Trump administration helped secure Brunson's release from captivity.

ANDREW BRUNSON, PASTOR IMPRISONED IN TURKEY, OPENS SENATE IN PRAYER, PRAYS OVER TRUMP

"I have worked hard to solve some of your problems," the president wrote. "Don't let the world down. You can make a great deal."

The president mentioned the letter during a press conference with Italian President Sergio Mattarella and stated he sent it to Erdogan, along with another letter from Gen. Mazloum Abdi, the Syrian Kurdish military leader who serves as commander of the Kurdish-led Syrian Democratic Forces (SDF).

In his letter, Trump said Mazloum "is willing to negotiate with you, and he is willing to make concessions that they would never have made in the past."

"History will look upon you favorably if you get this done the right and humane way," the president said, later adding: "I will call you later."

CLICK HERE TO GET THE FOX NEWS APP

Fox News' Jennifer Griffin, Lucas Tomlinson, Caleb Parke and Frank Miles contributed to this report.