Former Defense Secretary James Mattis, who led the Pentagon through the first two years of the Trump administration, warned during an exclusive interview with NBC's "Meet the Press" that the president's decision to pull troops from Syria's border in advance of a Turkish incursion could have dire consequences and lead to ISIS's resurgence.

"We have got to keep the pressure on ISIS so they don't recover," Mattis said in response to "Meet the Press" moderator Chuck Todd's question on whether the United States would regret Trump's decision.

"We may want a war over; we may even declare it over. You can pull your troops out as President Obama learned the hard way out of Iraq, but the 'enemy gets the vote,' we say in the military. And in this case, if we don't keep the pressure on, then ISIS will resurge. It's absolutely a given that they will come back."

Defense Secretary Mark Esper said Sunday that about 1,000 U.S. troops were preparing to withdraw from northeastern Syria as hundreds of Islamic State group supporters escaped from a displacement camp. There were reports of alleged atrocities as Turkish forces continued their advance.

The troops will leave the area "as safely and quickly as possible," Esper told CBS' "Face the Nation," according to an excerpt of an interview Sunday, adding that they will not leave the country entirely.

Trump announced the initial decision to move U.S. forces from the region of northern Syria bordering Turkey last week, stating that Turkey wanted to begin an operation in the region to resettle Syrian refugees.

The move drew widespread and intense criticism, including from some of Trump's Republican colleagues, because Turkey's offensive is against Kurdish-led Syrian Democratic Forces (SDF), who have been a key U.S. ally in the fight against ISIS.

Mustafa Bali, the official SDF spokesperson, has told NBC News the U.S. decision to stand aside as Turkey moves into Syria was "shocking and unexpected." Bali called it "a huge mistake" that will allow ISIS to "reunite itself and appear again stronger than before."

"It seems that the policy of the United States is to betray their friends and allies," Bali added.

The Trump administration announced Friday that it was prepared to slap stiff economic sanctions on Turkey in response to the country's military offensive.

A day before, Trump defended his decision by stating on Twitter that he was "trying to end the ENDLESS WARS."

"Turkey has been planning to attack the Kurds for a long time. They have been fighting forever. We have no soldiers or Military anywhere near the attack area. I am trying to end the ENDLESS WARS," the president said.

Trump said in an additional series of tweets that the United States had "defeated 100% of the ISIS Caliphate."

"We did our job perfectly!" he wrote.

Mattis, a retired Marine Corps general who led U.S. Central Command and served in the Gulf War, and the wars in Afghanistan and Iraq, called the situation on the ground one “of disarray.”

"Obviously, the Kurds are adapting to the Turkish attacks,” he said. “And we'll have to see if they're able to maintain the fight against ISIS."

When asked about whether the attacks could hurt the morale of American troops who served alongside those Kurds in the fight against ISIS, Mattis said leaders must keep a watchful eye on morale.

“We have to always be concerned with this. We have got to have policies that make sense on the front line, mostly, that make sense in terms of the defense of the country,” he said.

“They've signed a blank check, payable to the American people with their lives, to defend this experiment that you and I call America. Their views on, on matters of policy are important to us. And how do we bring them onboard and keep their heart and soul committed to the mission? It's always critical.”

Trump’s relationship with Mattis, his first secretary of defense, ultimately turned sour in the waning days before his December resignation.

Mattis announced his resignation in a letter where he extolled the importance of America’s alliances and argued for a “resolute and unambiguous” approach to nations like China and Russia, whose “strategic interests are increasingly in tension with ours.”

“Because you have a right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position,” Mattis wrote.

While Mattis’ letter said he’d resign at the end of February, he was quickly replaced by Jan. 1.

Since his resignation, Mattis has chosen his words about Trump carefully, arguing that the military works best when it stays out of politics.

“I won’t make political assessments right now. The military’s job is to protect this experiment, this America,” he said.

“I have a lot of faith in the American people. They know how to vote. They don't need military generals telling them that they think this political assessment is the one they should go with, or the other one is, that sort of thing, especially as corrosive as the political debate has grown in the country. This would be the worst time, I think, for military people to step out like that.”