FILE - President Trump gets a briefing from his senior military leaders, including Defense Secretary James Mattis (L), in the Cabinet Room at the White House, October 23, 2018.
FILE - President Trump gets a briefing from his senior military leaders, including Defense Secretary James Mattis (L), in the Cabinet Room at the White House, October 23, 2018.

U.S. Secretary of Defense Jim Mattis Thursday announced he was quitting, personally handing his letter of resignation to U.S. President Donald Trump following a lunch meeting at the White House.
 
While not mentioning Trump by name, the letter from Mattis outlined sharp differences between his views and those of the president, notably on the importance of allies and the use of U.S. power.
 
“We must be resolute and unambiguous in our approach to those countries whose strategic interests are increasingly in tension with ours,” Mattis wrote, warning that Russia and China in particular "want to shape a world consistent with their authoritarian model-gaining veto authority over other nations' economic, diplomatic and security decisions."

FILE - U.S. Secretary of Defense Jim Mattis gives a news conference after a NATO defense ministers meeting at the Alliance headquarters in Brussels, Belgium, June 29, 2017.
NATO Allies Fearful About Departure of 'Trusted' Mattis  

For two years, U.S. Defense Secretary Jim Mattis has been urging nervous NATO counterparts to judge the Trump administration by its actions and not by the president's tweets or off-the-cuff remarks. But on Thursday, President Donald Trump posted a tweet the Europeans couldn't ignore: an announcement that the four-star general would soon be leaving the Pentagon.  

The tweet wasn't altogether a surprise, coming a day after Trump ordered U.S. ground troops out of northern Syria, but it has shocked America's traditional allies, who like Mattis are eager to preserve the U.S.-European alliance.

“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,” Mattis concluded, saying he would step down at the end of February.
 
The defense secretary’s decision came one day after Trump announced he would withdraw some 2,000 U.S. troops from Syria, a move the Pentagon opposed. 
 
Mattis did not mention the dispute over Syria in his letter, but he did note his "core belief" that U.S. strength is "inextricably linked" with the nation's alliances with other countries. 
 
President Trump first announced Mattis's departure on Twitter, saying the former four-star Marine general will retire "with distinction."
 
"During Jim's tenure, tremendous progress has been made, especially with respect to the purchase of new fighting equipment. 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!"


White House Press Secretary Sarah Sanders told reporters late Thursday that Trump and Mattis are on good terms despite not agreeing on foreign policy and other issues. 
 
"He and the president have a good relationship, but sometimes they disagree," Sanders said. "That doesn't mean you don't have a good relationship with somebody. He was laying out the reasons he was stepping down from his post."
 
Still, the resignation has sparked an outpouring of anger and despair from both Republican and Democratic lawmakers, and even top U.S. officials.
 
“I was deeply saddened,” U.S. Director of National Intelligence Dan Coats said in an official statement Friday, describing Mattis as a "national treasure."
 
"The experience and sound judgement that Secretary Mattis has brought to our decision-making process is invaluable,” Coats continued. “His leadership of our military won the admiration of our troops and respect of our allies and adversaries." 
 
Much of the pushback from U.S. officials and lawmakers has centered on the decision to withdraw U.S. troops from the fight against the Islamic State terror group in Syria – a decision that, according to some officials, ultimately convinced Mattis to resign.

U.S.-backed forces have made steady progress against Islamic State over the past several years. Last week, taking advantage of a dramatic increase in U.S. and coalition airstrikes, the forces were able to enter the town of Hajin, part of the terror group’s last stronghold in eastern Syria.
 
But despite Trump’s declaration of victory against IS, senior administration officials have said it will be up to the U.S. partner forces to liberate the rest of Hajin and the surrounding areas, where about 2,000 IS fighters have been mounting a stubborn last stand for several months.
 
Pentagon officials have also warned that despite the gains, IS was still well-positioned to rebuild. And Mattis had said that before leaving, the U.S. must train enough local troops to assume the role of suppressing the militants. He said the United Nations peace process in Syria must progress toward a resolution of the country’s eight-year-old civil war.
 
While a relatively small number of troops are involved, their withdrawal will have sweeping consequences in Syria's long-running civil war. Allies will be more heavily burdened with confronting energized adversaries and Turkey, Iran and Russia's influence in Syria will increase.

FILE - U.S. forces are seen at the Kurdish People's Protection Units (YPG) headquarters after it was hit by Turkish airstrikes in Mount Karachok near Malikiya, Syria, April 25, 2017.

"This is scary," said Senate Intelligence Committee Vice Chairman Mark Warner, a Democrat. "Secretary Mattis has been an island of stability amidst the chaos of the Trump administration."
 
Republican senator and former presidential hopeful Marco Rubio tweeted, "It makes it abundantly clear that we are headed toward a series of grave policy errors which will endanger our nation, damage our alliances and empower our adversaries."
 
While the decision to pull out of Syria may have been the last straw for Mattis, tensions have been simmering over other issues for quite some time, including on Russia and Iran.
 
Mattis believed Russian President Vladimir Putin has been trying to undermine NATO and assaulting Western democracies.
 
"[Putin's] actions are designed not to challenge our arms at this point, but to undercut and compromise our belief in our ideals," Mattis told U.S. Naval War College graduates at a commencement ceremony in June.

美国国防部长马蒂斯在美国海军战争å­
FILE - Defense Secretary James Mattis addresses U.S. Naval War College class of 2018 graduates during a commencement ceremony in Newport, R.I., June 15, 2018.

 
But Trump has praised Putin's leadership skills and recently caused concern among U.S. allies by calling for Russia's reinstatement in the group of major industrial nations. Russia was expelled from what was then the Group of Eight after Moscow's annexation of Crimea from Ukraine.
 
Another point of contention between the two men involved the Iran nuclear deal.
 
Mattis argued the U.S. should consider staying in the Iran nuclear deal unless Tehran was found not to be abiding by the agreement. Iran was following the pact's rules, according to the International Atomic Energy Agency, which monitors the use of nuclear energy and has verified Iranian compliance with the accord multiple times since 2015.
 
Despite Mattis's position, Trump pulled out of the deal in May, saying it had been poorly negotiated during the administration of former President Barack Obama.
 
As Mattis turned in his resignation, the Defense Department was preparing plans to withdraw up to half of the 14,000 U.S. troops in Afghanistan in the coming months, U.S. officials said. The development marks a sharp departure from the Trump administration's policy to force the Taliban to the negotiating table after more than 17 years of war.

U.S. troops listen to a security briefing before leave their base in Logar province, Afghanistan, August 5, 2018.
US Weighs Sizable Drawdown in Afghanistan, Officials Say

President Donald Trump is considering withdrawing roughly half of the more than 14,000 U.S. troops stationed in Afghanistan, senior administration officials say.

Under the reported plan, about 7,000 U.S. troops would start coming home in January, and the rest would exit in the coming months in a phased drawdown. 

There was no comment from the Pentagon or U.S. Central Command.

The U.S. troops are part of a non-combat NATO force whose primary mission is training and advising Afghan forces in taking over responsibility for security in their country.

The comments from the U.S.

Rumors of Mattis leaving the Defense Department have been circulating for months.
 
In October, Trump appeared on the television news show 60 Minutes, where he told TV anchor Lesley Stahl that while "I like General Mattis," he believed he knew more about NATO than his defense secretary. 
 
"I think he's sort of a Democrat, if you wanna know the truth," Trump said. "But General Mattis is a good guy. We get along very well. He may leave. I mean, at some point, everybody leaves. Everybody. People leave. That's Washington."
 
Mattis became secretary of defense shortly after Trump's inauguration and is one of the longest-serving Cabinet members.
 
Before that, Mattis served 44 years in the Marine Corps and led the Marines and British troops during the bloody Battle of Fallujah in Iraq in 2004.