Mattis announced plans to resign in a candid resignation letter to Trump that laid bare the growing divide between them, and implicitly criticized Trump’s disregard for America’s closest allies. He released the letter after a face-to-face meeting with Trump in which the two men also aired their differences, a senior White House official said according to Reuters.

“Because you have a right to a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position,” Mattis said in the letter.

US officials said the resignation had not been forced by Trump.

Trump announced on Wednesday that US troops in Syria would be withdrawn, a decision that upended American policy in the region. On Thursday, officials said the president was considering defying Mattis again by ordering a substantial US pullout from the 17-year-long conflict in Afghanistan.

Mattis, a retired Marine general whose embrace of NATO and America’s traditional alliances often put him at odds with Trump, had advised against the Syria withdrawal. One official said it was a contributing factor to his resignation.

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