Donald Trump got "rolled" by Turkish President Recep Tayyip Erdogan, a National Security Council source with direct knowledge of the discussions told Newsweek.

In a scheduled phone call on Sunday afternoon between President Trump and President Erdogan, Trump said he would withdraw U.S. forces from northern Syria. The phone call was scheduled after Turkey announced it was planning to invade Syria, and hours after Erdogan reinforced his army units at the Syrian-Turkish border and issued his strongest threat to launch a military incursion, according to the National Security Council official to whom Newsweek spoke on condition of anonymity.

The U.S. withdrawal plays into the hands of the Islamic State group, Damascus and Moscow, and the announcement left Trump's own Defense Department "completely stunned," said Pentagon officials. Turkey, like the United States, wants regime change in Syria. Russia and Iran support the Assad regime.

"President Trump was definitely out-negotiated and only endorsed the troop withdraw to make it look like we are getting something—but we are not getting something," the National Security Council source told Newsweek. "The U.S. national security has entered a state of increased danger for decades to come because the president has no spine and that's the bottom line."

Newsweek granted the National Security Council official anonymity because they were not authorized to discuss the matter publicly. The source said it would not be surprising to see a Turkish incursion in the next 24 to 96 hours.

Turkey has long considered the Kurdish militia in Syria to be a terrorist insurgency, despite the United States providing military and financial aid to the group in its fight against ISIS, the Islamic State militant group. A battle with the vastly superior military of Turkey, a NATO ally, could drive the Kurds into the arms of Bashar Al-Assad, the Syrian dictator that Washington wants ousted, and by extension into an alliance with Russia and Iran, two U.S. rivals with forces in Syria.

The White House said late Sunday evening in a statement that Turkey will soon invade northern Syria but both the Defense Department and Trump on Twitter said they made clear to Turkey that they do not endorse a Turkish operation in northern Syria.

"As I have stated strongly before, and just to reiterate, if Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate the Economy of Turkey (I've done before!)," said Trump on Twitter Monday. "They must, with Europe and others, watch over the captured ISIS fighters and families...it is time now for others in the region, some of great wealth, to protect their own territory."

According to the NSC official, who had first-hand knowledge of the phone call, Trump did not endorse any Turkish military operation against Kurdish Forces, but also did not threaten economic sanctions during the phone call if Turkey decided to undertake offensive operations.

In a statement, White House Press Secretary Stephanie Grisham said, "The United States Armed Forces will not support or be involved in the operation, and United States forces, having defeated the ISIS territorial "Caliphate," will no longer be in the immediate area."

The New York Times reported Monday that about 100 to 150 American forces would withdraw from northern Syria but not completely from the country. Newsweek confirmed the Times reporting but the National Security Council official said the number was closer to 230 service members, among them U.S. Special Forces and reconnaissance units.

The Times also reported witnesses observed United States forces withdraw from two observation posts in Tel Abyad and Ein Eissa in northeastern Syria. Newsweek confirmed these on Monday—a senior Defense Department official said American forces are about 90 percent complete with the withdraw as of publication.

One of the main issues in the phone call between the two world leaders concerned the roughly 2,000 Islamic State militant prisoners being held by the Kurdish-dominated Syrian Defense Forces in northeastern Syria, who the U.S. military assists financially.

Trump told Erdogan he did not want anything to do with ISIS prisoners despite the United States not currently detaining Islamic State prisoners in Syria. The Syrian Defense Forces control custody of the prisoners.

Erdogan said Turkey would take custody of the ISIS militant prisoners, according to the White House statement and the National Security Council official Newsweek spoke to for this story.

"The ISIS prisoners, some of them, will eventually be freed amongst the chaos, and remain in the area or go elsewhere to rejoin the fight," speculated the National Security Council official.

The White House statement on Sunday also expressed Trump's long-held frustration with how other NATO-allied countries had dealt with captured Islamic State group fighters. The statement singled out "France, Germany and other European nations," for refusing to take back their citizens who had joined the Islamic State militant group.

U.S. Army General Mark A. Milley, Trump's newly minted chairman of the joint chiefs of staff, is scheduled to meet with the service chiefs this morning to discuss the matter, said National Security Council source.

The National Security Council official said they could not speak about what Trump's admirals and generals may plan, but said they would not expect anything out of the usual norms as the Defense Department follows orders with or without advanced knowledge.

If the United States had refused to move out of Turkey's war path, U.S options would not just be the threat of potential conflict between nation-state militaries, it would have been applied pressure on the Turkish economy, according to the National Security Council official.

However, the United States chose not to stand its ground to protect Kurdish Forces against Turkish airstrikes as a part of Trump's "America First policy" and his historical views that war is bad for business, according to the official.

On Sunday, Erdogan reinforced his army units at the Syrian-Turkish border hours after he issued his strongest threat to launch Turkish forces over the border and into the "buffer zone," between the Tigris and Euphrates rivers.

The buffer zone—sometimes referred to as the peace corridor—was established to prevent another Turkish invasion of northern Syria. The United States had been working to establish a proper demilitarized zone prior to Trump's endorsement of Turkey's military plans.

If Trump's withdraw of U.S. forces had not occurred, the National Security Council official told Newsweek, the United States could have continued to refine the buffer zone on the Syrian-Turkish border.

"To be honest with you, it would be better for the United States to support a Kurdish nation across Turkey, Syria and Iraq," said the National Security Council official. "It would be another Israel in the region."

The current foreign policy debacle is what prompted Trump's former defense secretary, James Mattis, to resign his post after Trump decided to abruptly withdraw American forces from Syria in December 2018. Mattis was the last of the generals touted as the "adults" in the administration—and was an outspoken opponent of a Syrian withdrawal.

A senior Defense Department official told Newsweek in January no U.S. general was happy with the decision to pull back U.S. troops from Syria as Pentagon officials feared the withdrawal could spark an ISIS resurgence similar to the Taliban's growing influence and territory in Afghanistan.

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Administration officials in January told Newsweek Trump's sudden withdraw order could undercut strategic U.S. alliances with regional allies; free Russia and Iran to re-establish a full military presence and solid footing in the Mediterranean; and leave U.S.-backed Kurdish fighters vulnerable to being decimated by a Turkish air campaign.

A complete withdrawal could also potentially give up a valuable regional position to American military forces that threaten United States interests in the region, including the interests of allies such as Israel and, to some extent, Jordan.

The National Security Council official compared Turkey to "playground bullies" on Monday.

"When the bigger guy [United States] moves aside in the playground, they [Turkey] get to beat on the smaller guy [Syrian Defense Forces] and this is not about the U.S. being the world police," the National Security Council source told Newsweek.

"We are telling the world, we will use you and then throw you away," the official added. "It's not like they don't have a television in Asia, in Africa, and South America."