Washington (Agencies) US Defence Secretary Jim Mattis on Thursday resigned over policy differences with President Donald Trump who ordered a complete troop pullout from Syria and a significant withdrawal from Afghanistan.
Trump steadfastly defended his sudden push for retrenchment, vowing that the United States would no longer be the "policeman of the Middle East" and saying the 2,000-strong US force in Syria was no longer needed as the Islamic State group had been defeated.
Mattis, a battle-hardened retired four-star general seen as a moderating force on the often impulsive president, made little attempt to hide his disagreements with Trump.
"Because you have the right to have a secretary of defense whose views are better aligned with yours," Mattis said in a letter to Trump, "I believe it is right for me to step down from my position."
Mattis hailed the coalition to defeat the Islamic State as well as NATO, the nearly 70-year-old alliance between North America and Europe whose cost-effectiveness has been questioned by the businessman turned president.
One day after the surprise announcement on Syria, a US official told foreign news agency that Trump had also decided on a "significant withdrawal" in a much larger US operation -- Afghanistan.
Some 14,000 troops are fighting the Taliban in Afghanistan as part of the longest-ever US war, launched in response to the September 11, 2001 attacks.
Trump has surrounded himself with former military men and shown an uncharacteristic public deference toward Mattis, a bookish 68-year-old who has disagreed with the president behind the scenes on issues from Russia to Iran to accepting transgender soldiers.
He hinted at Mattis's departure as far back as October, telling CBS: "It could be that he is (leaving). I think he's sort of a Democrat, if you want to know the truth... He may leave. I mean, at some point, everybody leaves."
On Twitter Thursday, however, Trump had only praise for his defense secretary, who will serve until the end of February, crediting him with achieving "tremendous progress."
- 'National security crisis' -
US lawmakers across the political spectrum voiced concern over a rebirth of the Islamic State group in Syria and sounded an alarm as Mattis unmoors from the unpredictable administration.
Senator Marco Rubio, a member of Trump's Republican Party, said Mattis in his letter "makes it abundantly clear that we are headed towards a series of grave policy errors which will endanger our nation, damage our alliances and empower our adversaries."
Democratic Senator Mark Warner called Mattis "an island of stability amidst the chaos of the Trump administration" and voiced fears of policy driven by "the president's erratic whims."
Trump was quick to note that he has ramped up military spending, but he has been most interested in deploying troops at home to carry out his key domestic goal of cracking down on unauthorized immigration.
"Does the USA want to be the Policeman of the Middle East, getting NOTHING but spending precious lives and trillions of dollars protecting others who, in almost all cases, do not appreciate what we are doing? Do we want to be there forever? Time for others to finally fight," he tweeted.
- Putin praises Trump -
The US withdrawal will make Russia, which has deployed its air power in support of President Bashar al-Assad, the pre-eminent global power in the Syrian conflict.
"The fact that the US has decided to withdraw its troops is right," Russian President Vladimir Putin said during an annual year-end press conference, saying that "on the whole I agree with the US president" on the level of damage inflicted on Islamic State.
Putin, who has described the fall of the Soviet Union as a historic geopolitical disaster, sees Moscow's longtime ally Syria as a key asset in preserving influence in the Middle East.
Iran's Shiite clerical regime has also strongly backed Assad, a secular leader from the heterodox Alawite sect.
Turkey opposes Assad and may be emboldened by Trump to attack Kurdish fighters inside Syria, who fought alongside US troops against the Islamic State group.
Turkey links the Kurds who dominate the US-backed Syrian Democratic Forces to a decades-old insurgency at home, but had been reluctant to strike for fear of setting off a crisis if the United States suffered casualties.
Mustefa Bali, a spokesman for the Syrian Democratic Forces, said the fighters would keep up the battle against Islamic State -- but that all bets were off if Turkey attacks.
- Worries in Europe -
Bali said the Kurdish forces would keep locked up the Islamic State extremists in their custody -- but alleged that Turkey may target prisons to sow chaos once US troops leave.
French President Emmanuel Macron's office called the prisoner issue "extremely important for France" and urged discussions to avoid "one of the negative consequences of a hasty retreat."
The Islamic State movement has claimed credit for a slew of bloody attacks around the world, including the 2015 coordinated assault on Paris, and experts estimate that thousands of sympathizers remain.
Meanwhile Germany, which has taken in more than one million refugees stemming in large part from the Syria conflict, questioned Trump's assessment that the threat was over.
While fighting has largely subsided in Syria and the Islamic State group holds little territory, a political solution remains elusive in ending the war that has killed more than 360,000 and displaced millions since 2011.
On Afghanistan, Trump's decision comes days after a US representative, Zalmay Khalilzad, held a fresh meeting in Abu Dhabi with the Taliban.
But he voiced doubts about the Taliban's sincerity after the militants refused to meet with negotiators from the government in Kabul.