Brett McGurk, the special presidential envoy to the coalition fighting the Islamic State, has accelerated his resignation, telling colleagues this weekend that he could not in good conscience carry out President Trump’s newly declared policy of withdrawing American troops from Syria.

Mr. McGurk, a seasoned diplomat who was considered by many to be the glue holding together the sprawling international coalition fighting the terrorist group, was supposed to retire in February. But according to an email he sent his staff, he decided to move his departure forward to Dec. 31 after Mr. Trump did not heed his own commanders and blindsided America’s allies in the region by abruptly ordering the withdrawal of the 2,000 troops.

His decision comes right after the departure of Secretary of Defense Jim Mattis, whose own resignation letter was seen as a rebuke of the president’s actions in the region.

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