US President Donald Trump's announcement late Sunday night that the US would withdraw troops from northeastern Syria was a stunning betrayal of the US's Kurdish partner forces in the fight against ISIS, military leaders and experts say.

Kurdish forces have sustained nearly 11,000 casualties in the fight against ISIS; now, they prepare for a new battlefront against a Turkish invasion into Syrian territory they control, while the US watches.

"The value of an American handshake is depreciating," Brett McGurk, who resigned from his post as Special Presidential Envoy for the Global Coalition to Counter ISIL last December over Trump's announcement that he would pull all US troops from Syria, tweeted.

"Trump today said we could 'crush ISIS again' if it regenerated," McGurk wrote. "With who? What allies would sign up? Who would fight on his assurances?"

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Kurdish fighters led the assault on ISIS strongholds across Syria alongside US special operations troops, brutally attacking entrenched ISIS fighters at a devastating cost.

President Donald Trump's decision to pull US troops out of a so-called "safe zone" in northeastern Syria leaves these forces and their compatriots at the mercy of Turkish forces and President Recep Tayyip Erdogan, who has made no secret of his desire to launch an invasion there. Military leaders say Trump's abandonment of an ally in the fight against ISIS is a grave mistake.

"This policy abandonment threatens to undo five years' worth of fighting against ISIS and will severely damage American credibility and reliability in any future fights where we need strong allies," retired Gen. Joseph Votel, the former head of US Central Command who oversaw the campaign against ISIS, wrote in The Atlantic on Tuesday.

The US is abandoning the Kurdish elements of the Syrian Democratic Forces to fight against Turkish forces, who threaten their territory from the north. Turkey views the Kurdish fighters as terrorists, an extension of the Kurdish Worker's Party (PKK) in Turkey, which has fought back — sometimes violently — against state-sanctioned oppression there.

As Votel outlines in The Atlantic, the Kurdish forces were the only suitable regional allies in the fight against ISIS — the only group sufficiently trained and strategically savvy enough to fight alongside the US and defeat ISIS's territorial caliphate, which it did in a series of stunning victories.

Kurdish forces liberated "tens of thousands of square miles and millions of people from the grip of ISIS," as Votel says. In return, they sustained 11,000 casualties and lost their American buffer against the threat of Turkish invasion.

'The value of an American handshake is depreciating.'

When Trump originally declared that he was pulling US troops from Syria in late 2018, Secretary of Defense James Mattis resigned, saying the president deserved a defense secretary who shared his views.

In his recent book "Call Sign Chaos: Learning To Lead," Mattis wrote of his decision to resign: "When my concrete solutions and strategic advice, especially keeping faith with allies, no longer resonated, it was time to resign, despite the limitless joy I felt serving alongside our troops in defense of our Constitution" — the allies, in this case, reportedly being the Kurds.

"The consequences of such unreliability from the Oval will reverberate well beyond Syria," Brett McGurk, the former Special Presidential Envoy for the Global Coalition to Counter ISIL, tweeted Monday in response to Trump's abrupt decision.

"The value of an American handshake is depreciating," continued McGurk, who resigned from his post last December over Trump's announcement that he would pull all US troops from Syria.

In abandoning Kurdish forces to the will of Turkey, the US also decreases regional stability and risks gains against ISIS. As a Pentagon Inspector General report from August found, ISIS is resurging in Iraq and Syria; now, with Kurdish attention focused on fighting Turkey at the border, approximately 11,000 ISIS militants in makeshift prisons guarded by SDF fighters are that much more likely to break out of captivity and wreak havoc, Votel writes.

But as McGurk wrote, "Trump today said we could 'crush ISIS again' if it regenerated. With who? What allies would sign up? Who would fight on his assurances?"