This story has been updated.

WASHINGTON — Defense Secretary Jim Mattis resigned Thursday as the Pentagon chief just one day after President Donald Trump abruptly announced he was pulling all American troops out of Syria against the advice of his top national security advisers.

Mattis cited differences in views with the president in a resignation letter laced with criticism of Trump’s foreign policy. The letter was distributed by Pentagon officials moments after Trump announced Mattis’ impending “retirement, with distinction” via Twitter late Thursday afternoon.

“You have the right to have a secretary of defense whose views better align with yours,” Mattis wrote in the letter indicating he would leave at the end of February. “I believe it is right for me to step down from my position.”

Mattis’ resignation sent shockwaves through the halls of the Pentagon, where officials on Friday declined to speak on the record but privately expressed admiration for their outgoing leader, who has been seen for the last two years as a stable presence in Trump’s chaotic presidency. The outgoing defense chief delivered the news to his staff Thursday shortly after returning from a meeting with Trump.

It was Trump’s unilateral decision to remove the roughly 2,000 American troops in Syria who are aiding a Kurdish-led militia of anti-Islamic State fighters that proved the final straw in Mattis’ tenure as Trump’s defense secretary, a defense official said Friday. With his resignation letter in hand, Mattis marched to the White House on Thursday afternoon and made one last effort before resigning to convince the president to reverse his Syria decision.

Mattis’ resignation caught Trump by surprise, the official said on the condition of anonymity. It ended months of speculation among Washington officials that the retired four-star Marine general had lost his influence with the president, who he routinely disagreed with on major matters of foreign policy. The announcement also came just as news broke that Trump intended to remove half of the roughly 14,000 American troops in Afghanistan now supporting the fight against the Taliban, ISIS and al-Qaida.

Pentagon officials on Friday declined to confirm the reports on Afghanistan, referring all questions to the White House.

Mattis, who is beloved by American servicemembers in cult-like fashion for his warrior monk persona, cited in his letter the importance of the United States backing its key allies, which was viewed by officials as a rebuke of Trump’s decision to pull support in Syria from the Kurdish-led militia. The group, the Syrian Democratic Forces, continues to fight ISIS but also faces threat of an assault by the Turkish military, a NATO ally of the United States who views the Kurds as terrorists.

“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,” Mattis wrote. “While the [United States] 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.”

Mattis, 68, took a low-profile approach to his job as defense secretary, avoiding television appearances and routinely touting the Pentagon as an apolitical institution focused solely on defending the United States. Despite their differences, Mattis never publicly stated displeasure with any of the president's policies until releasing his letter Thursday.

He spent much of his tenure as defense secretary visiting with allies at the Pentagon and around the world, delivering a message that the United States remained steadfast in its support for them, despite Trump’s rhetoric and insistence on an “America First” approach to international relations. Mattis traveled frequently, visiting with his defense counterparts and heads of state across Europe and Asia.

But Mattis and Trump disagreed on a plethora of issues from the start of their relationship. From their first meeting in the weeks after Trump’s November 2016 election victory, Mattis and Trump found they did not agree on much. The two spent the bulk of the encounter discussing their differing opinions on topics, including the use of torture in interrogation and support for NATO, according to officials familiar with the engagement. But at the meeting, before Mattis was officially offered the top Pentagon job, he was able to convince Trump not to support torture or drastically alter the U.S. relationship with NATO.

However, during the last two years, Trump has repeatedly blindsided Mattis and other Pentagon officials with apparent spur-of-the-moment policy decisions with major implications for the U.S. military.

Trump caught the Pentagon by surprise in July 2017 when he tweeted he would reinstate a ban on transgender men and women serving in the military, a position that defense officials have said Mattis opposed.

More recently, Mattis initially opposed Trump’s decisions to stall major training exercises in South Korea amid nuclear negotiations with North Korea, to build a new military branch focused on space operations and the deployment of thousands of military troops to the U.S.-Mexico border. The defense chief also advised Trump privately to no avail to remain in the Iran nuclear deal and the Paris Climate Agreement.

But it was Trump’s decision on Syria – and perhaps on Afghanistan as well – that proved Mattis’ final straw, officials said.

Trump on Wednesday announced he had made the decision to remove the troops operating in Syria and end their support of the Kurdish-led militia fighting the remnants of ISIS in the eastern region of the war-torn country. He proclaimed victory against ISIS, even as fighting continued and defense officials estimated the terrorist group still boasted thousands of fighters in Syria and Iraq.

On Friday, Trump tweeted he had “done more damage to ISIS than all recent presidents....not even close!”

Mattis had previously denied he had any intention of leaving his post, saying he was squarely focused on his No. 1 priority – increasing the strength and killing power of the U.S. military and preparing its servicemembers to fight a near-peer adversary such as Russia or China, which he sees as rising powers competing with the United States for international dominance.

But Trump, who had once routinely touted Mattis as one of “my generals” using the nickname Mad Dog, had signaled in October that the relationship had soured as he told CBS' "60 Minutes" that he thought the defense secretary was “sort of a Democrat, if you want to know the truth.”

Defense officials insisted Thursday evening that Trump had not requested Mattis’ resignation, and the president on Twitter praised his two-year tenure as defense secretary.

“During Jim’s tenure, tremendous progress has been made, especially with respect to the purchase of new fighting equipment,” the president wrote. “General Mattis was a great help to me in getting allies and other countries to pay their share of military obligations. A new Secretary of Defense will be named shortly. I greatly thank Jim for his service!”

Reactions to the news came swiftly Thursday evening.

"A Secretary of Defense quitting over a public disagreement with a President whose foreign policy he believes has gone off the rails is a national security crisis. No way around it,” tweeted Sen. Chris Murphy, D-Conn. “Just imagine the morale crisis at the Department of Defense right now, having sustained the 1-2 punch of the Syria reversal and Mattis resignation. Devastating.”

Rep. Adam Kinzinger, R-Ill., praised Mattis’ decision, just a day after expressing deep displeasure with Trump’s Syria decision.

"I admire Secretary Mattis,” he wrote on Twitter. “His service has been one of honor [and] respect. His resignation is an act of patriotism, standing for our American principles above all else.”

In Europe, Mattis’ resignation is likely to rattle allies in NATO. In Asia, officials worried long-standing alliances with nations such as South Korea could be stressed.

“Mattis departure deeply worrisome. Very challenging days ahead for the Pentagon and the nation,” tweeted retired Navy Adm. Jim Stavridis, a former NATO supreme allied commander.

While Trump has long been skeptical of the alliance's value, Mattis is one of its biggest boosters.

Trump’s choice of Mattis to lead his Pentagon after the 2016 election was also widely applauded by military officials and Republican and Democratic lawmakers. He was easily confirmed to the post in a 98-1 Senate vote, just hours after Trump was sworn into office.

His selection required a waiver approved by Congress because he had not yet been out of uniform for seven years, as required by law.

Mattis, from Richland, Wash., was a career Marine Corps infantry officer, whose tenure in uniform spanned four decades. He led Marines as a lieutenant colonel in Iraq in 1991. Later as a general, he commanded forces in Afghanistan shortly after the 9/11 terrorist attacks and later commanded forces again in Iraq.

He retired from the Marine Corps in 2013 after spending three years leading U.S. Central Command, which oversees the ongoing wars in the Middle East.

In his resignation letter, Mattis did not thank or offer any praise for Trump, stressing instead his admiration for the men and women of the U.S. military and his devotion to national service.

"I have been privileged to serve as our country’s 26th Secretary of Defense which has allowed me to serve alongside our men and women of the Department in defense of our citizens and our ideals,” Mattis wrote. “… I very much appreciate this opportunity to serve the nation and our men and women in uniform.”

Stars and Stripes reporters Claudia Grisales, John Vandiver and Caitlin M. Kenney contributed to this report.

dickstein.corey@stripes.com Twitter: @CDicksteinDC

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