US Defense Secretary James Mattis resigned on Thursday on the heels of President Donald Trump's plans to withdraw troops from Syria, citing irreconcilable policy differences in a move that took Washington by surprise.

“Because you have the right to have 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 wrote in his letter to the President.

Earlier on Thursday, a senior administration official told CNN’s Jake Tapper that Mattis was “vehemently opposed” to the Syria decision and a possible Afghanistan troop withdrawal.

Indeed, Mattis’ resignation letter amounts to a rebuke of several of Trump’s foreign policy views, with the outgoing defense secretary touting the importance of US alliances and of being “unambiguous” in approaching adversaries such as Russia and China. It is devoid of any praise for the President.

The resignation emerged at a chaotic moment in Trump’s presidency: The US government is teetering on the edge of a government shutdown, the Trump administration is about to face the hot light of Democratic investigations and the President is grappling with the fallout of a series of firings and resignations. Trump, seeking to downplay the news, stepped out in front of Mattis’ resignation, spinning it as a retirement.

Mattis did not explicitly cite his opposition to the President’s planned withdrawal of US troops from Syria — which caught US allies off guard — but the retired four-star general was privately adamant in urging Trump against the pullback.

It was just the latest issue on which Mattis has sought to position himself as a bulwark against some of the President’s rashest decisions, but his relationship with the President has grown increasingly fractured in recent months and his efforts to deter Trump on key issues less influential.

The President touted the “tremendous progress” that has been made during Mattis’ tenure at the helm of the Defense Department and thanked him for his service.

Trump said a successor “will be named shortly.”

Mattis is the latest senior administration to leave Trump’s Cabinet, after former Attorney General Jeff Sessions was pushed out the day after the midterm elections in November, former Secretary of State Rex Tillerson was unceremoniously fired in March and national security adviser H.R. McMaster was replaced earlier this year.

Mattis’ future was brought into question most recently after Trump, in an October interview with CBS’ “60 Minutes,” labeled him as “sort of a Democrat,” adding that the retired general “may leave” and that “at some point, everybody leaves.”

Mattis, a bachelor, has been described as a “warrior monk,” married to the Marines. He served in the first Gulf War and in Afghanistan. He once led the all-important US Central Command, which is in charge of the US military in the Middle East. He was also supreme allied commander of NATO.

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