It was not immediately clear how far American forces would pull back, as the Defense Department had yet to finalize the safe zone area with Turkey. "The US does not endorse Turkish military action in northern Syria,” a US defense official told Al-Monitor on Monday, despite a White House statement on Sunday indicating that US troops would stand aside as Ankara launches an attack.

As US troop elements have begun to pull back from Syria’s contested border with Turkey, a separate defense official said Defense Secretary Mark Esper will be “reaching out to affected allies today,” though it was not clear whether the Pentagon would be contacting the SDF, who criticized the American pullback as a betrayal on Sunday.

“Certainly not a surprise to most, the [Department of Defense] opposed the decision to allow the Turkish operation,” a senior administration official told Al-Monitor. The official added, “We have spent considerable time and effort to build the safe zone under the expressed idea that it would appease the Turks and prevent an incursion. The Turks just used it as a way to conduct reconnaissance for their operation. That was completely deceptive on their part.”

In internal US administration discussions before the White House announced Trump’s decision, Pentagon officials argued that Turkey had used negotiations for a northeast ‘safe zone’ to scout a military operation against the US-backed Syrian Democratic Forces, known as the SDF.

The Pentagon fought back against President Donald Trump’s surprise decision to remove US troops from northeast Syria to allow a Turkish operation, a senior administration official told Al-Monitor on Monday.

But the public reaction from the Pentagon appeared to mask more nuanced conversations within the administration, as the SDF cited the move as American disloyalty to the group, which said it suffered 11,000 dead in the fight against the Islamic State (IS).

The Pentagon knew that Trump's call with Erdogan was coming on Sunday night, the senior administration official said, but Defense Department leaders did not know the president would green-light the Turkish incursion. During the call on Sunday, the Turkish leader accepted an invitation from Trump to visit the White House next month.

The Defense Department had consistently told Turkish leaders it was completely opposed to an intervention, which is why the agency had supported the buffer zone and joint patrols. Trump is set to be briefed by senior military leaders on Monday afternoon.

The senior administration official, who spoke to Al-Monitor on condition of anonymity to discuss internal US deliberations, said the move would harm relationships with future partners and take away “one of the few successes we have had in the Middle East” in the four-year campaign to defeat IS.

The move “put in jeopardy all of the success we have had in the Defeat ISIS campaign,” the official said, adding, “It betrays the trust of one of our best partners we have had on this almost 20-year fight against extremists, and a whole generation of US military professionals will remember it as just that — a betrayal.”

The Pentagon has approximately 1,000 US troops in Syria, including a company-sized element that sits astride an Iranian supply line near Iraq and Jordan. Esper and Chairman of the Joint Chiefs of Staff Gen. Mark Milley spent much of the weekend on the phone with Turkish Defense Minister Hulusi Akar, the official added.

Anticipating a Turkish incursion on Monday, the Pentagon pulled Turkey off of the air-tasking order, which would make it harder to conduct missions in the region, and intelligence feeds run through the joint operations center set up with the US in the safe zone, said Pentagon Spokesperson Lt. Col. Carla Gleason.

Despite pushback from Democrats and Republican allies in Congress, President Trump sought to firmly state that his decision would deter US adversaries such as Russia and Iran in a series of tweets on Monday.

Al-Monitor previously reported in April that the small presence of American troops in Syria relies heavily on the Kurdish-dominated SDF to provide escorts, force protection and intelligence about IS fighters on the ground.

By abandoning the safe zone, former US officials say the Trump administration is acknowledging that an incursion by Ankara would likely threaten the residual American presence in Syria.

“If Turkey invades, US forces would have to leave given [their] reliance on [the] SDF for mobility and force protection,” said Dana Stroul, a former Democratic staffer for the Senate Foreign Relations Committee and co-chair of the congressionally mandated Syria Study Group.

Trump’s decision marks another major reversal in US policy in Syria, after first announcing an American withdrawal last December. As recently as Saturday, Defense Secretary Mark Esper told reporters he “agreed” with Turkish counterpart Hulusi Akar “that we need to make the security mechanism work.”

Sunday’s decision is a sign that despite the Pentagon’s protests, Trump’s instincts to pull out of Middle East conflicts will continue to dominate US national security decisions.

“[The Department of Defense] and others in the administration kept the United States in Syria for a while, but those voices are either gone or quiescent now,” said Alexander Bick, a former National Security Council director for Syria during the Barack Obama administration. “I think Trump is more confident calling the shots now — especially when he thinks he's in tune with his political base, which needs something right now.”