Defense Secretary Jim Mattis on Sunday signed the order to withdraw an estimated 2,200 U.S. forces from Syria, the Pentagon said.

The Syria withdrawal, which was directed last week by President Donald Trump, was reported to be one of the main reasons that Mattis submitted his letter of resignation last Thursday.

In his letter, Mattis cited the need to maintain critical alliances and partnerships, a reference that defense analysts in the days following have interpreted to not only mean the country’s traditional NATO allies but also its Syrian Kurdish partners who have been instrumental to the coalition’s fight against the Islamic State.

Some of those same Syrian Kurdish forces are now at risk of being targeted by Turkey, which views them as belonging to terrorist organizations.

On Monday, the Associated Press reported that Turkish-backed Syrian fighters are preparing to deploy in eastern Syria alongside Turkish troops once American forces withdraw. The AP also reported that a spokesman said that the forces have begun massing on the front line of a town held by Kurdish-led forces.

Iraqi troops could deploy into neighboring Syria, Iraq’s prime minister said Monday, according to another AP report. Adel Abdul-Mahdi said his government was “considering all the options” to protect Iraq from threats across its borders, where Iraq is keeping reinforcements to guard against infiltration by Islamic State militants.

On Sunday, just days after stating that U.S. troops would begin to return home immediately, Trump said he’d spoken with Turkey’s president Recep Erdogan to coordinate a “slow and highly coordinated pullout.”

“The execute order for Syria has been signed,” a defense official told Military Times in a statement. “I am not going to provide more operational details at this time.”

Military officials said they were still in the dark about how a drawdown would take place, or what the timeline would be. Marine Corps Commandant Gen. Robert Neller, traveling overseas to visit troops during the holidays, was asked by Marines he visited what they could expect next.

"The honest answer is I have no idea,” Neller said, according to the Wall Street Journal.

Trump on Sunday decided to counter Mattis' resignation by announcing that he would be replaced by acting secretary of defense Patrick Shanahan on Jan. 1, far earlier than Mattis had planned to go. This will be Mattis' final week on the job.