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Defense Secretary James Mattis resigned 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.

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 like Russia and China. It is devoid of any praise for the President.

The news 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 the resignation, spinning Mattis’ resignation as a retirement.

Mattis did not explicitly cite his opposition to the President’s planned withdrawal of US troops in 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 Mattis’ efforts to deter Trump on key issues less influential.

In his letter, Mattis pointedly stated that the strength of the US depends in part on the strength of its alliances around the globe, many of which have become notably frayed under Trump.

“One core belief I have always held is that our strength as a nation is inextricably linked to the strength of our unique and comprehensive system of alliances and partnerships. While the US remains the indispensable nation in the free world, we cannot protect our interests or serve that role effectively without maintaining strong alliances and showing respect to those allies,” he added.

How it happened

Mattis met with Trump one-on-one in the Oval Office, a senior White House official told CNN’s Kaitlan Collins. Mattis told Trump he was going to be leaving and offered his resignation letter.

“They had differences on some issues,” the official said, but couldn’t say if it was over Syria. “Just over the course of the last couple of months,” this person added.

Trump first announced Mattis’ departure in a tweet.

“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,” Trump tweeted.

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

Trump said a successor “will be named shortly.”

Mattis and the President’s other top national security advisers opposed Trump’s decision to withdraw from Syria.