WASHINGTON (AP) — Defense Secretary Jim Mattis resigned 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."

Mattis went to the White House on Thursday with his resignation letter in hand to meet with the president and spoke to Trump for about 45 minutes, according to a senior U.S. official familiar with the incident but speaking on conditions of anonymity to discuss a private meeting.

There was no confrontation between the two men, the official said, and there was no one issue that caused the resignation. However, the official said, Syria likely was the last straw for Mattis.

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.