Brett McGurk, the seasoned diplomat who brokered and led the delicate coalition of partners in the global fight against ISIS, has resigned from the high-profile post, becoming the latest member of the Trump administration to leave in protest at the president's abrupt decision to withdraw U.S. troops from Syria.

The decision by McGurk, the special presidential envoy for the Global Coalition to Defeat ISIS, comes days after Defense Secretary Gen. James Mattis quit after Trump declared Wednesday that the fight against the terror group is over and he would pull 2,000 American soldiers from Syria.

McGurk, considered to be the United State’s institutional authority on Syria and Iraq, has served in senior diplomatic roles in the region under three presidents. He was initially expected to leave his current role in February, but resigned on Friday, reportedly informing Secretary of State Mike Pompeo that he would leave immediately because he disagrees with the president's assessments of the war against ISIS.

“The recent decision by the president came as a shock and was a complete reversal of policy that was articulated to us. It left our coalition partners confused and our fighting partners bewildered,” McGurk said in an email to his staff.

“I worked this week to help manage some of the fallout but—as many of you heard in my meetings and phone calls—I ultimately concluded that I could not carry out these new instructions and maintain my integrity,” he also reportedly wrote.

According to McGurk’s State Department bio, he “leads a global coalition of 79 members and helps coordinate all aspects of U.S. policy related to the ultimate destruction of ISIS.”

McGurk was reportedly in the region meeting with allies in the anti-ISIS fight, such as Kurdish leader Masoud Barzani, and was caught off-guard by Trump’s announcement. Trump’s snap proclamation not only flew in the face of advice from his top national security aides, but came without consultation with the United States’ allies.

The removal of U.S. troops effectively leaves America’s allies to fend for themselves in the war-torn region. The president’s prior policy had been to keep U.S. forces in Syria until Iran withdrew its militias, and to bolster stabilization efforts carried out by America’s allies in the region, according to CBS News.

McGurk’s resignation came one day after Mattis quit because of deep disagreements with President Trump, such as the United State’s handling of military alliances.

McGurk’s resignation officially goes into effect on Dec. 31, CBS said.

He had apparently expressed concern to Barzani over Kurds in Syria, such as the Kurdish Syrian Democratic Forces. SDF troops have been on-the-ground fighters in Syria against ISIS, and have been bolstered by the United States, including with military advisers and weapons, as well as air strikes.The fate of SDF troops is uncertain if U.S. troops leave Syria, as it’s unclear whether the United States will keep providing air support to their anti-ISIS efforts, CBS notes.

The withdrawal also comes shortly after McGurk promised that the U.S. was committed to keep fighting ISIS. While ISIS only held 1 percent of the area it once controlled because of the U.S.-led coalition forces, McGurk said the group still posed a threat.

“I think it's fair to say Americans will remain on the ground after the physical defeat of the caliphate, until we have the pieces in place to ensure that that defeat is enduring,” McGurk reportedly said, elaborating that “it would be reckless if we were just to say, well, the physical caliphate is defeated, so we can just leave now. I think anyone who's looked at a conflict like this would agree with that.”

McGurk also tweeted on Dec. 10 “We salute the Iraqi forces and #Peshmerga that fought heroically against #ISISand continue to attack its clandestine cells. The US and our global @coaltion will continue to support Iraq and its people to ensure this historic military victory over ISIS is lasting & permanent.”

McGurk, who is expected to take a position at Stanford in spring 2019, has been a long-time hand on issues related to the Middle East, especially Iraq under presidents George W. Bush, Barack Obama and Trump.

His accomplishments include the brokering of the Status of Forces Agreement in 2008, a deal which set parameters for the U.S. presence in Iraq as well as its ultimate withdrawal. He is also seen as one of the chief architects “the surge,” a Bush strategy to reduce violence in Iraq.

He was poised to become the U.S. Ambassador to Iraq in 2012.

The Obama pick withdrew from his nomination, however, after an email discussion of “blue balls” between him and a Wall Street Journal reporter surfaced, revealing that they had an affair.

He nevertheless headed U.S. efforts to fight ISIS since 2015, making him one of the few Obama appointees asked to stay on in the Trump administration, CBS notes.