0627 GMT January 18, 2020
Mattis announced on Thursday that he would resign from the post at the end of February, writing in a letter to Trump that he deserved a secretary whose views were “better aligned” with the US president.
The resignation of the Pentagon chief came after Trump confirmed he was withdrawing US troops from Syria despite opposition from allies and top US military officials.
The US president has also ordered the start of pulling out some 7,000 troops from Afghanistan, about half of the total number of American boots on the ground in the war-torn country, Presstv Reported.
Senate Democratic Leader Charles Schumer described Mattis as one of a few symbols of strength and stability in the US administration.
"President Trump is plunging the country into chaos. The market's down another 500 points, General Mattis is stepping down and we know he has real disagreements with the president in Syria and on the wall,” Schumer said.
“Everything that indicates stability, everything that indicates strength, everything that indicates knowledge is leaving this administration,” he added, referring to the departures of Mattis and White House Chief of Staff John Kelley.
‘I am shaken' by resignation of Mattis
Democratic Representative Nancy Pelosi, the Minority Leader of the House of Representatives, told reporters on Capitol Hill that she was “shaken” by the news of Mattis’ resignation.
“I’m shaken by the news because of the patriot that Secretary Mattis is. I think everybody in the country should read his letter of resignation,” she said.
"I am shaken by the resignation of General Mattis. For what it means to our country, for the message it sends to our troops, and for the indication of what his view is of the commander-in-chief.
Pelosi also called Mattis “a comfort to many of us as a voice of stability in the Trump administration.”
Senate Republican Majority Leader Mitch McConnell said he was "distressed" by the US defense secretary’s resignation.
“I believe it’s essential that the United States maintain and strengthen the post-World War II alliances that have been carefully built by leaders in both parties. We must also maintain a clear-eyed understanding of our friends and foes, and recognize that nations like Russia are among the latter," McConnell said in a statement.
“So I was sorry to learn that Secretary Mattis, who shares those clear principles, will soon depart the administration. But I am particularly distressed that he is resigning due to sharp differences with the president on these and other key aspects of America’s global leadership," he added.
McConnell concluded his statement by asking Trump to consider a replacement whose views aligned with those of Mattis's.
“It is regrettable that the president must now choose a new Secretary of Defense," McConnell wrote. "But I urge him to select a leader who shares Secretary Mattis’s understanding of these vital principles and his total commitment to America’s service members."
Mattis's departure is 'scary' and 'bad news'
Reacting to the resignation news, Democratic Senator Mark Warner, vice chairman of the Senate Intelligence Committee, stated that the Trump administration would be destabilized by the defense secretary's departure.
"This is scary. Secretary Mattis has been an island of stability amidst the chaos of the Trump administration," he wrote on Twitter. "As we’ve seen with the President’s haphazard approach to Syria, our national defense is too important to be subjected to the President’s erratic whims."
Republican Senator Lindsey Graham said the resignation announcement filled him with "great sadness."
"It is with great sadness that I was informed of the resignation of General Mattis," he said in a tweet. "He is one of the great military leaders in American history. He should be proud of the service he has rendered to President @realDonaldTrump and our nation."
Senator Chris Murphy, a Democratic from the US state of Connecticut, denounced Thursday's news as “devastating.”
“Just imagine the morale crisis at the Department of Defense right now, having sustained the 1-2 punch of the Syria reversal and Mattis resignation,” he tweeted.
Senator Marco Rubio, a Republican from Florida, said Mattis's resignation made it clear that America is headed for a "series of grave policy errors which will endanger our nation, damage our alliances & empower our adversaries."
"I hope we who have supported this administrations initiatives over the last two years can persuade the President to choose a different direction," Rubio added. "But we must also fulfill our constitutional duty to conduct oversight over the policies of the executive branch."
Democratic Senator Dick Durbin said Washington has lost the "adult in the room when the President goes off on a tweet-filled, uninformed rant about our national security."
Mattis joins a long list of former Trump administration senior figures who have either quit or been removed, some unceremoniously like Secretary of State Rex Tillerson who Trump fired via Twitter in March.
Trump's White House has had the highest turnover of senior-level staff of the past five presidents, according to the Brookings Institution think tank.
Trump has announced that he will nominate a successor to Mattis shortly.