(AP) — Defense Secretary Jim Mattis resigned Thursday after clashing with President Donald Trump over the abrupt withdrawal of U.S. troops from Syria and after two years of deep disagreements over America’s role in the world.

Mattis, perhaps the most respected foreign policy official in Trump’s administration, will leave by the end of February after two tumultuous years struggling to soften and moderate the president’s hardline and sometimes sharply changing policies. He told Trump in a letter that he was leaving because “you have a right to have a Secretary of Defense whose views are better aligned with yours.”

His departure was immediately lamented by foreign policy hands and lawmakers on both sides of the aisle, who viewed the retired Marine general as a sober voice of experience in the ear of a president who had never held political office or served in the military. Even Trump allies expressed fear over Mattis’ decision to quit, believing him to be an important moderating force on the president.

“Just read Gen. Mattis resignation letter ,” tweeted Florida Sen. Marco Rubio. “It makes it abundantly clear that we are headed toward a series of grave policy errors which will endanger our nation, damage our alliances & empower our adversaries.”