US President Donald Trump has sacked Secretary of State Rex Tillerson, replacing him with the director of the CIA, Mike Pompeo.
Thanking Mr Tillerson for his service on Twitter, Mr Trump said the new state secretary would do "a fantastic job".
Mr Tillerson, a former chief executive of ExxonMobil, was only appointed to the job just over a year ago.
The president also nominated Gina Haspel to become the first woman director of the CIA.
The Department of State said Mr Tillerson had not spoken to the president and was "unaware of the reason" for his firing.
Under Secretary Steve Goldstein said: "The Secretary had every intention of staying because of the critical progress made in national security."
Speaking to reporters outside the White House on Tuesday, Mr Trump said his differences with Mr Tillerson came down to personal "chemistry".
"We got along actually quite well, but we disagreed on things," said the president.
"When you look at the Iran deal, I think it's terrible. I guess he thought it was OK.
"I wanted to either break it or do something and he felt a little bit differently, so we were not really thinking the same.
"With Mike, Mike Pompeo, we have a very similar thought process. I think it's going to go very well.
"Rex is a very good man I like Rex a lot."
Trump quickly soured on Tillerson
Rex Tillerson, who as a long-time corporate executive was an unconventional pick for secretary of state, seemed like he was on thin ice from very early on in his administration tenure.
He was distrusted by veteran State Department employees, who viewed him as an outside interloper with little affinity for the organisation he headed. The president, initially enamoured with the brash Texan, quickly appeared to sour on his pick, as he frequently broke with the White House line on foreign policy.
Reportedly calling the president a moron (intensified by a choice expletive) certainly didn't help, either.
Mr Tillerson did manage to hang around through those earlier controversies, outliving his earlier expectations that he would be gone by year's end. Instead he made it to March, but it was clear that this was a move some time in the making, given that Mr Trump already had successor Mike Pompeo lined up to take over the reins.
Just last week, Mr Trump tweeted that he had a few more staff changes he wanted to make. Now it's getting clearer that he had one move in particular in mind.
Mr Tillerson was on an official tour of Africa last week when he was apparently caught unawares by Mr Trump's announcement that he would hold talks with North Korean leader Kim Jong-un.
The top US diplomat's team said he was feeling unwell on Saturday and later in the weekend the state department said he would cut short his tour by a day.
On Monday, Mr Tillerson appeared to depart from White House talking points when he backed British authorities in blaming the Kremlin for the poisoning of a former Russian spy near his home in southern England.
The secretary of state said the nerve agent attack "clearly came from Russia" and "certainly will trigger a response".
But earlier in the day, the White House declined to point the finger at Russia.
Reports have swirled of a schism in the Trump administration between the commander-in-chief and his top diplomat, as the US faces a host of vexatious foreign policy conundrums, from North Korea to Iran.
Last October, Mr Tillerson was forced to convene a news conference to deny reports that he was considering quitting, though he did not comment on a report that he had called his boss a moron after a meeting last July at the Pentagon.
Last autumn, Mr Trump publicly undercut the former Texas oilman by tweeting that he was "wasting his time" trying to negotiate with nuclear-armed North Korea.
Mr Tillerson was reported to be astonished at how little Mr Trump grasped the basics of foreign policy.
The New York Times quoted sources as saying Mr Trump was irritated by Mr Tillerson's body language during meetings.
Mr Tillerson was said to roll his eyes or slouch when he disagreed with the decisions of his boss.