WASHINGTON (NYTIMES, BLOOMBERG) - James Mattis, the four-star Marine general turned United States defence secretary, resigned on Thursday (Dec 20) in protest of President Donald Trump's decision to withdraw 2,000 US troops from Syria, where they have been fighting the Islamic State in Iraq and Syria (ISIS).

Mr Trump announced the resignation in two tweets on Thursday evening, and said Mr Mattis will leave at the end of February.

Officials said Mr Mattis went to the White House on Thursday afternoon in a last attempt to convince Mr Trump to keep US troops in Syria. He was rebuffed, and told the President that he was resigning as a result.

Hours later, the Pentagon released Mr Mattis' resignation letter, in which he implicitly criticised his commander-in-chief.

Mr Mattis said in the letter that he believes that the President deserves a defence secretary who is more in tune with his worldview.

"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," Mr Mattis wrote.

"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 from my position," he wrote.

His departure leaves the Trump administration without one of the few officials viewed as standing between a mercurial President and global tumult.

The President said he would name Mr Mattis' replacement shortly.

The President's tweets announcing the departure of his defence secretary shocked officials at the Pentagon, who as recently as Thursday afternoon were insisting that Mr Mattis had no intention of resigning his post, despite his anger at Mr Trump's decision, announced on Wednesday, to withdraw US troops from Syria.

Mr Trump had also said in a Tweet on Thursday that the US should not be the policeman of the Middle East, a point Mr Mattis made clear he agreed with even as he emphasised the principle of collective defense undergirding the post-WWII security framework, to which the President frequently gives short shrift.

Presidential adviser Stephen Miller backed President Donald Trump's decision to pull troops out of Syria in an interview with CNN.

Mr Miller echoed the President's comments that ISIS had been defeated in Syria, while putting the onus on Russia, Turkey and Syria to crush any future extremism.

"ISIS has been defeated," Mr Miller told CNN's Wolf Blitzer on "The Situation Room".

"But if ISIS wants to retrench and regrow and reorganize, it's going to be up to those countries to defeat their enemy."

"ISIS is the enemy of Russia, ISIS is the enemy of Assad, ISIS is the enemy of Turkey," Mr Miller said in the CNN interview.

"Are we supposed to stay in Syria for generation after generation, spilling American blood to fight the enemies of all those countries?"

Mr Mattis had told close friends that he would continue in the job despite his deteriorating relationship with Mr Trump, because he viewed his commitment to protecting the Defence Department and its 1.3 million active-duty service members as paramount.

The widely accepted narrative that Mr Mattis was the adult in the room when at the White House came to annoy Mr Trump. In October, the President accused Mr Mattis of being a Democrat - a charge akin to treason in the current Republican administration.

His resignation also came after US officials raised the possibility that President Trump would order a drawdown of US troops from Afghanistan.

As defence secretary, Mr Mattis oversaw the world's most powerful military, supervising active-duty troops based in the US and deployed worldwide, including in war zones in Afghanistan, Iraq and on the Saudi border with Yemen.

There are also around 25,000 US troops in South Korea, where they have served for generations as a deterrent against North Korea. As with Mr Trump's abrupt firing of Mr Jeff Sessions, the attorney general, the split with Mr Mattis was a full turn in a relationship that once appeared strong.

In Mr Mattis' early days as defence 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 the US' 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 Defence Department red tape.

Over the past six months, the President and the defence chief have also found themselves at odds over Nato policy, whether to resume large-scale military exercises with South Korea and, privately, whether Mr Trump's decision to withdraw the US from the Iran nuclear deal has proved effective.

Mr Mattis was long seen as a force for stability in foreign policy in an administration that has had to manage crises from North Korea to Syria under a President who prides himself on his unpredictability. But like other senior national security advisers, he was believed to have pushed back on Mr Trump's decision this week to abruptly withdraw American troops from Syria.

After Mr Trump said he'd nominate Mr Mattis for the top Pentagon job, the late Senator John McCain hailed him as "one of the finest military officers of his generation and an extraordinary leader who inspires a rare and special admiration of his troops". Short and wiry with a brush-cut haircut, Mr Mattis was known as the "Warrior Monk" and sometimes as "Mad Dog", a nickname he disliked as much as Mr Trump loved invoking it.

At the Pentagon, he followed a succession of defence chiefs - Mr Ash Carter, Mr Chuck Hagel and Mr Leon Panetta - who each lasted about two years in office.

DEFENCE SPENDING SURGE

Beyond his reputation as a voice for stability in military and foreign policy, Mr Mattis may be best-remembered for overseeing a surge in defence spending. The fiscal year 2019 military budget of more than US$715 billion (S$980 billion) bore his imprint and was seen as fulfilment of a key campaign pledge by Mr Trump, ramping up spending for weapons systems, including Lockheed Martin's F-35 jet and new aircraft carriers built by Huntington Ingalls.

"He managed to secure large budget increases for the Pentagon while simultaneously dissuading the President from withdrawing troops from the wars in Afghanistan, Iraq and Syria, as he advocated during his campaign, and in fact convinced him to step up efforts in these areas," said Mr Rob Levinson, a defence analyst with Bloomberg Government.

"He sought to increase the Pentagon's 'lethality' and tame its sprawling bureaucracy and associated costs, but the jury is still out on whether these efforts have made any real difference."

Mr Mattis found himself on the defensive after excerpts from author Bob Woodward's book Fear, published in early September, painted the publicly taciturn Pentagon secretary as critical in private of the commander-in-chief.

According to Mr Woodward, Mr Mattis pushed back early in 2018 when Mr Trump questioned the massive US military presence on the Korean peninsula, asking why the US was spending so much.

'WORLD WAR III'

"We're doing this in order to prevent World War III," Mr Mattis said, according to the book.

He went on to tell associates that the President had the understanding of a "fifth or sixth-grader", according to Mr Woodward. It also quoted the secretary as saying that defence chiefs "don't always get to choose the president they work for".

Mr Mattis was one of the first top administration officials to deny the book's claims.

"The contemptuous words about the President attributed to me in Woodward's book were never uttered by me or in my presence," Mr Mattis said in a Sept 4 statement.

"While I generally enjoy reading fiction, this is a uniquely Washington brand of literature, and his anonymous sources do not lend credibility."

But Mr Trump's most direct public criticism of Mr Mattis followed the book's revelations.

"I think he's sort of a Democrat, if you want to know the truth," Mr Trump said in an interview on 60 Minutes in October. "He may leave. I mean, at some point, everybody leaves. Everybody. People leave. That's Washington."

He walked that back a bit in November, telling reporters who asked if he would remove Mr Mattis after the midterm election, "no".

Mr Mattis was a defence secretary of few words in public. Asked in a May 2017 interview on CBS News about what keeps him up at night, he responded: "Nothing. I keep other people awake."

But he occasionally let slip an intemperate remark during his military career. In 2005 during a speech in San Diego, Mr Mattis was quoted as saying it was "fun to shoot some people", that "it's a lot of fun to fight", and "it's a hell of a hoot".

Even before he took charge of the Pentagon, Mr Mattis and Mr Trump were in sync in their views on Iran. Mr Mattis called the Iran nuclear deal reached under President Barack Obama "an arms control agreement that fell short" and labelled the regime in Teheran "the single most enduring threat to stability and peace in the Middle East".

Mr Mattis wasn't always in lockstep with Mr Trump. Like other foreign policy officials in the Trump administration, he spoke out against Russia's meddling in the 2016 campaign and its actions in Syria and Ukraine.

The National Defence Strategy published during Mr Mattis' tenure sought to focus US strategy towards "great power" relations with Russia and China, putting less emphasis on the fight against terrorism that came to dominate US thinking after the Sept 11, 2001 attacks.

In an administration infamous for leaks and turmoil, Mr Mattis' reticence to create controversy generated extra scrutiny the few times he appeared to subtly criticise the political climate in the US.

In August 2017, video surfaced of Mr Mattis telling troops in Jordan that the US "has problems that we don't have in the military. Just hold the line until our country gets back to understanding and respecting each other".

Mr Trump credited the blunt-talking Mr Mattis with getting him to rethink his campaign pledge to revive waterboarding, an interrogation tactic used against suspected terrorists that Mr Obama had banned.

"He said, 'I've always found, give me a pack of cigarettes and a couple of beers and I'll do better,"' Mr Trump recounted to the New York Times.

Mr Mattis retired from the military in 2013 after a 41-year career in the Marines that took him from rifleman to head of US Central Command.

A native of Pullman, Washington, Mr Mattis had more than 30 years' experience in the Middle East, where he first deployed in 1979 as an infantry company commander.

Later he led Nato's transformation office and rewrote - along with Army General David Petraeus - the military's counterinsurgency field manual. He was one of few military officers who identified US economic debt as a national security issue.