The top US special envoy to the coalition to fight the ISIS terrorist group, Brett McGurk, has quit his post, announced the State Department Saturday. His resignation comes just after Donald Trump abruptly ordered the withdrawal of US troops from Syria, prompting the resignation of Defense Secretary Jim Mattis, who cited key disagreements with the US president. Just last week McGurk said "nobody is declaring a mission accomplished" in the battle against ISIS -- just days before the presidents stunning announcement of victory against the terror group. Trump -- who postponed his holiday vacation as failed budget talks triggered a partial US government shutdown -- again on Saturday said "ISIS is largely defeated." "When I became President, ISIS was going wild," the president tweeted. "Now ISIS is largely defeated and other local countries, including Turkey, should be able to easily take care of whatever remains. Were coming home!" McGurk reportedly said in his resignation letter that ISIS terrorists were in fact not defeated, and that prematurely withdrawing US troops could foster conditions allowing the extremists to amass power in the region once more. The 45-year-old top envoy was set to leave his position in February, but reportedly felt he could no longer continue in the job after Trumps declaration. His resignation is effective December 31. McGurk was appointed by former President Barack Obama in October 2015 and has been instrumental in shaping Washington’s policy in northern Syria, particularly its backing of Kurdish-led Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab factions that have been fighting ISIS with US support for three years. The troop pullout will leave thousands of Kurdish fighters vulnerable to Turkish attack. "It would be reckless if we were just to say, Well, the physical caliphate is defeated, so we can just leave now," McGurk had told journalists earlier this month. "I think anyone whos looked at a conflict like this would agree with that." McGurk has served as the US envoy to the Global Coalition to Defeat ISIS since 2015. He also served as the deputy assistant secretary of state for Iraq and Iran, and worked under Republican George W. Bush as a senior official on Iraq and Afghanistan. In an email announcing his decision to his colleagues obtained by The New York Times, McGurk called Trumps move "a shock" and "a complete reversal of policy that was articulated to us." "It left our coalition partners confused and our fighting partners bewildered," he said, according to the paper. "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." In addition to the Syria decision, Trump blindsided politicians and international allies with plans to slash troop numbers in Afghanistan. The momentous reversal of years of US foreign policy will leave the war-torn regions at risk of continued and potentially heightened bloodshed. Heavyweight adviser Mattis -- a decorated Marine general often referred to as "the last adult in the room" -- made clear in his resignation letter that pulling out of Syria crossed the line. The departures of Mattis and now McGurk follow those of National Security Advisor H.R. McMaster and White House chief of staff John Kelly -- leaving Trump, who has no political, diplomatic or military experience, increasingly alone. On Friday White House press secretary Sarah Sanders dismissed concerns over the slew of resignations and Trumps isolation. "At the end of the day, the American people elected one person to be the commander-in-chief," she said. "And at the end of the day, he makes the decision," Sanders said. "Thats what he was elected to do."