The US Defence Secretary James Mattis is to leave his post at the end of February next year.
In a tweet Donald Trump said: "General Jim Mattis will be retiring, with distinction, at the end of February, after having served my Administration as Secretary of Defense for the past two years."
He added that Gen Mattis' replacement will be named shortly.
It appears that defence secretary has decided to leave after clashing with the president.
The announcement comes a day after the president announced that US troops were to pull out of Syria, much to the surprise of Congress, US allies around the world and, apparently, the Pentagon.
A White House official has all but admitted Gen Mattis and Mr Trump argued over the change in policy, saying that in a meeting on Thursday the pair had a "difference of opinion on some issues", prompting Gen Mattis to quit.
In his resignation letter, the general made it clear there were deep divisions between himself and Mr Trump.
In it he says the commander-in-chief deserved a defence secretary "whose views are better aligned with yours".
He said that his "core belief" is that the United States needs to maintain strong alliances and show allies respect and that it should be unambiguous with nations like Russia and China.
The resignation of the former Marine general had been widely anticipated in Washington since Mr Trump's Syria decision.
In fact speculation about his future as defence secretary had been growing since October when Mr Trump described him as "sort of Democrat" in a CBS interview and hinted that he might be leaving.
Gen Mattis was a career soldier, rising through the ranks of the Marine Corps. He served in the first Gulf War, Afghanistan, and Iraq.
From 2007 to 2010, he commanded the United States Joint Forces Command and was commander of United States Central Command from 2010 to 2013.
After retiring from the military he worked in the private sector before being nominated as defence secretary in January 2017.
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During his time in the role he has been seen to be at odds with his boss on several occasions, consistently stressing Russia's threat to the world order and voicing his disagreement with the proposed withdrawal from the Iran nuclear deal.
In stepping down he joins the long list of Mr Trump administration figures who have either resigned or been sacked.
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