US Defence Secretary James Mattis will be retiring “with distinction” at the end of February, President Donald Trump announced on Thursday.
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It comes a day after the president’s controversial announcement that all US troops would be withdrawn from Syria.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.
Responding to the news, Republican Senator Lindsey Graham said Gen Mattis “should be proud of the service he has rendered to President [Trump] and our nation”.
“He has been in the fight against radical Islam for decades and provided sound and ethical military advice to President Trump,” he tweeted.
Democratic Senator Mark Warner said the development was “scary”, describing Gen Mattis as “an island of stability amidst the chaos of the Trump administration”.
What did Trump say about Syria?
Mr Trump announced his decision to withdraw some 2,000 US troops from Syria on Wednesday, asserting that the Islamic State (IS) group had been defeated there.
READ: EFCC impostor, two others bag five-year jail term
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.
President Trump, who has long pledged to pull troops out of Syria, said on Twitter that it was time to bring them home after their “historic victories”.
In a later tweet, he defended his decision and said it should not have come as a surprise.
Getting out of Syria was no surprise. I’ve been campaigning on it for years, and six months ago, when I very publicly wanted to do it, I agreed to stay longer. Russia, Iran, Syria & others are the local enemy of ISIS. We were doing there work. Time to come home & rebuild. #MAGA
— Donald J. Trump (@realDonaldTrump) December 20, 2018
US officials quoted by Reuters news agency say that in addition, air attacks against IS will cease.
The White House would not give a timescale for the withdrawal but defence officials quoted by the New York Times said President Trump wanted it done within 30 days.
