"You can pull your troops out, as President [Barack] Obama learned the hard way, out of Iraq, but the 'enemy gets the vote,' as we say in the military. And in this case, if we don't keep the pressure on them, ISIS will resurge. It's — it's absolutely a given that they will come back," Mattis continued.

Trump's move was widely lambasted by both parties in Congress. A bipartisan group of House lawmakers is drafting a resolution to impose punitive sanctions on Turkey for its move.

Late Friday, a U.S. military outpost came under fire as the Turkish military launched air and artillery strikes. Defense Secretary Mark Esper warned Turkey that it must protect U.S. military personnel.

Turkey’s military said Saturday it captured Ras al-Ayn, one of the largest border towns in Syria, marking one of the biggest gains since the invasion began, according to The Associated Press.

Mattis resigned in December 2018, days after Trump first announced the pullout of American troops from Syria, a decision that was partly reversed.

Trump has long called for drawing down the U.S. military's commitments in the Middle East and around the world, making the issue a key part of his presidential campaign.

"The Endless Wars Must End!" Trump tweeted Saturday afternoon.

Trump later called into "Justice with Judge Jeanine" on Fox News, saying Turkish President Recep Tayyip Erdogan was going into Syria whether or not troops were withdrawn.

"He was going to go in anyway. They’ve been fighting the Kurds for 200 years. He was going in anyway," Trump told host Jeanine Pirro.