WASHINGTON — Defense Secretary Jim Mattis resigned Thursday after clashing with President Donald Trump over the withdrawal of U.S. troops from Syria and after two years of 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 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.

Mattis did not mention the dispute over Syria in his letter or proposed deep cuts to U.S. forces in Afghanistan, another significant policy dispute. He noted his “core belief” that American strength is “inextricably linked” with the nation’s alliances with other countries, a position seemingly at odds with the “America First” policy of the president. The defense secretary also said China and Russia want to spread their “authoritarian model” and promote their interests at the expense of America and its allies. “That is why we must use all the tools of American power to provide for the common defense,” he wrote.