The US media reported that Secretary Mattis went to the White House on Thursday afternoon to persuade Trump to keep American troops in Syria but when he was rebuffed, he gave his resignation letter to the president.
Mattis, who embraced America's traditional alliances, said he was quitting after falling out with President Donald Trump over foreign policy, including surprise decisions this week to pull troops from Syria and start planning a drawdown in Afghanistan.
While the hiring process seems to take longer with each new opening, a few likely contenders for Defense Secretary are already waiting. The Senate GOP leader avoids critiquing the president and nearly always declines to answer questions about Trump's numerous controversial statements and blistering attacks against foes, including special counsel Robert Mueller. "It is imperative that he pick someone for the Pentagon - and frankly, clear out the rest of his national security team - and appoint people he can trust and whose views comport with his own".
In his resignation letter to the president, Mattis implicitly criticized him for not treating allies with respect and for not being "clear-eyed" about USA enemies and competitors.
Carl Bildt, a former prime minister of Sweden and a major voice in worldwide policy discussions, tweeted his dismay over the departure of Mattis. Lindsey Graham, R-S.C., told CNN that the secretary believes the "job in Syria is not yet done" despite Trump's announcement earlier this week that the United States will withdraw all troops from the country.
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Trump slammed Mattis as "sort of a Democrat" and hinted he may leave the administration.
Members of Congress also decried Mattis's departure, and some experts such as DeJonge Schulman are hoping Capitol Hill will step up to check Trump in Mattis's place.
"The resignation deprives Europe of one of its most reliable interlocutors and a firm supporter of the North Atlantic Treaty Organisation transatlantic alliance", Patrick Wintour, the diplomatic correspondent for the The Guardian wrote.
While there is much Mattis should not be proud of - sending troops to the border with Mexico as part of an election campaign show, not speaking out or resigning when Trump stood up for white supremacists at Charlottesville, attacked the Federal Bureau of Investigation or the press, or did any number of things that undermine American democracy and security - Mattis can be proud of taking a stand now.
Trump's decision to withdraw from the Iran nuclear deal is another glaring example of the differences between the two men. "As Secretary Mattis makes clear in his letter, President Trump's coziness with authoritarian regimes and his animosity to our closest allies are increasingly indefensible". It's about values - and, according to his letter, Mattis no longer believes the president shares his values. But it's hard not to respect that he just had the courage to do what no one else in the Trump administration has done - resign in protest and go public with his grave concerns about the danger that Trump poses to the country and the world.
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"If Trump does not get his way, and as his legal and political peril worsen in the coming weeks, it's not inconceivable that Trump might order a withdrawal of US forces from Korea", said Daniel Pinkston, a lecturer in worldwide relations at Troy University in Seoul.
In the end, Mattis resigned voluntarily, a senior USA defense official told FP.
United States officials said the resignation had not been forced by Trump.
"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 to Trump in his resignation letter. Trump threw that into doubt by not only threatening to levy auto tariffs against Japan, but questioning its contribution to security. Trump fired back at Allen through a tweet and said his fight against ISIS "failed badly".
Stephen Miller, a senior White House adviser, said Trump was entitled to a Pentagon chief with whom he could agree.
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Russian President Vladimir Putin listens during annual news conference in Moscow , Russia December 20, 2018. On the economy, Putin hailed another year of Russian growth after a previous period of stagnation.
"Deputy Secretary Shanahan remains exclusively focused on implementing the National Defense Strategy", Buccino said. Trump ultimately deferred to Mattis, who opposed the latter, signaling that he could be persuaded by his advisers.