US Defense Secretary Jim Mattis has abruptly said he is quitting after falling out with President Donald Trump over his foreign policies, one day after Trump rebuffed top advisers and decided to pull all US troops out of Syria.
Mattis announced plans to resign in a candid resignation letter to Trump that laid bare the growing divide between them, and implicitly criticised Trump's disregard for America's closest allies.
He released the letter after a face-to-face meeting with Trump on Thursday in which the two men also aired their differences, a senior White House official said.
"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Trump announced on Wednesday that US troops in Syria would be withdrawn, a decision that upended American policy in the region.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.
The news is certain to shock US military allies, already bewildered by what they see as Trump's unpredictable, go-it-alone approach to global security, and raises questions about whether Mattis' successor will be as steadfast about traditional treaty commitments, including to NATO.
When Mattis interviewed with Trump for the job in 2016, he split with the president-elect on a host of issues, including on NATO and the use of torture.
Trump ultimately deferred to Mattis, who opposed the latter.
But as time grew, Trump increasingly acted on his own instincts on a host of national security issues, choosing an "America First" agenda that contradicted Mattis' core beliefs.
Mattis' letter indicated he disagreed with Trump's isolationist policies, writing it was his belief the United States needed to maintain strong alliances and show allies respect.
Trump has withdrawn the United States from several international agreements since taking office in January 2017.
The Mattis resignation letter also said he believed the United States "must be resolute and unambiguous in our approach to those countries whose strategic interests are increasingly in tension with ours".
He identified Russia and China as countries that "want to shape a world consistent with their authoritarian model".
Trump, announcing Mattis' departure on Twitter, said he would nominate a successor soon.
"General Jim Mattis will be retiring, with distinction, at the end of February, after having served my Administration as Secretary of Defense for the past two years," he said.
One possible candidate to replace Mattis as defence chief could be Republican Senator Tom Cotton, long considered a frontrunner to eventually take the role.
Speculation Mattis might not last long in his post grew in October when Republican Trump said in a CBS interview the general was "sort of a Democrat" and might be leaving.
Mattis had argued for maintaining a strong US military presence in Afghanistan to bolster diplomatic peace efforts.
Source: Australian Associated Press