WASHINGTON-Defense Secretary Jim Mattis, whose experience and stability was widely seen as a balance to an unpredictable president, resigned Thursday in protest of President Trump's decision to withdraw American forces from Syria and his rejection of international alliances.
Mr. Mattis had repeatedly told friends and aides over recent months that he viewed his responsibility to protect the United States 1.3 million active-duty troops as worth the concessions necessary as defense secretary to a mercurial president.
But on Thursday, in an extraordinary rebuke of the president, he finally decided that Mr. Trump's decision to withdraw roughly 2,000 American troops from Syria was a step too far.
Officials said Mr. Mattis went to the White House on Thursday afternoon with his resignation letter already written, but nonetheless made a last attempt at persuading Mr. Trump to reverse his decision about Syria, which the president announced on Wednesday over the objections of his senior advisers.
Mr. Mattis, a retired four-star Marine general, was rebuffed. Returning to the Pentagon, he asked aides to print out 50 copies of his resignation letter and distribute them around the building.
Mr. Trump tweeted that General Mattis "was a great help to me in getting allies and other countries to pay their share of military obligations".
It comes a day after the president's controversial announcement that all US troops would be withdrawn from Syria.
Mr. Trump did not name a successor but said one would be appointed shortly. In his resignation letter, Gen Mattis described his views on "treating allies with respect" and using "all the tools of American power to provide for the common defence".
"Because you have the right to have a secretary of defence whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down," Gen Mattis wrote.
The news, which was met with strong criticism, is at odds with Mr. Mattis' position, who had warned that an early withdrawal from the country would be a "strategic blunder".
US troops have helped rid much of Syria's north-east of the jihadist group, but pockets of fighters remain.
The Pentagon said it was transitioning to the "next phase of the campaign" to eliminate IS but did not provide further details.
His resignation came as Congress appeared to be hurtling toward a government shutdown and a deep market slump became even worse over fears of continuing government turmoil.
With the ousting this month of fellow retired Marine Gen. John F. Kelly, the outgoing White House chief of staff, Mr. Mattis is the last to depart among the old guards of Mr. Trump's national security team — leaving it in the hands of Mike Pompeo, the president's second secretary of state, and John R. Bolton, the third White House national security adviser.
The resignation came as the Pentagon prepared to draw down forces in another global conflict.
Two Defense Department officials said about 7,000 troops will be withdrawn from Afghanistan in coming months, cutting in half the number of American forces there, in an early step to ending the United States' involvement in the 17-year war.
It was also the first resignation over a major national security issue by a leading cabinet member since 1980 when Cyrus Vance quit as secretary of state. Mr. Vance left over President Jimmy Carter's decision to attempt a rescue effort for American hostages in Iran that Mr. Vance considered ill-advised.
By starkly airing his disagreement with the president's way of doing business, Mr. Mattis made his departure count.
His complaints did not protest a single decision. Instead, the resignation letter condemned an approach to the world by Mr. Trump that the defense secretary views as destructive to American influence and power, and one that turns a blind eye to authoritarian governments.
He said the core of American national interests lay in "providing effective leadership to our alliances."
The relationship between Mr. Mattis and Mr. Trump had deteriorated for months. The widely accepted narrative that Mr. Mattis was the adult in the room when at the White House came to annoy Mr. Trump.
In Mr. Mattis's early days as defense secretary, he often ate dinner with Mr. Trump in the White House residence. Over hamburgers, and with the help of briefing folders, he explained to the president key points about America's relationships with allies.
But Mr. Mattis also quietly slow-walked a number of Mr. Trump's proposals, from banning transgender troops to starting a Space Force to putting on a costly military parade. In each case, he went through the motions of acquiescing to the White House — and then buried the plans in Defense Department red tape.