US President Donald Trump has sacked his national security adviser and replaced him with Fox News analyst and former UN Ambassador John Bolton.
Mr Bolton, 69, has long been a polarizing figure in Washington foreign policy circles.
He has previously has advocated using military force against Iran and North Korea, and has taken a hard line against Russia.
I am pleased to announce that, effective 4/9/18, @AmbJohnBolton will be my new National Security Advisor. I am very thankful for the service of General H.R. McMaster who has done an outstanding job & will always remain my friend. There will be an official contact handover on 4/9.— Donald J. Trump (@realDonaldTrump) March 22, 2018
As the State Department's top arms control official under President George W Bush, Mr Bolton was a leading advocate of the 2003 invasion of Iraq which was later found to have been based on bogus and exaggerated intelligence about Saddam Hussein having weapons of mass destruction and ties to terrorism.
In recent years, as a conservative media commentator, Mr Bolton has advocated hardline positions on stopping North Korea from getting nuclear weapons that could threaten the United States.
He has also advocated getting rid of the 2015 Iran nuclear deal, a pact Mr Trump has also heavily criticized.
Mr Trump announced that he was ousting his current national security adviser H.R. McMaster by tweet.
The White House said General McMaster "mutually agreed" with the president that he should resign. There has been increasing friction between the pair and reports he was planned to be replaced.
Last month Mr Trump chastised the General for telling a national security conference in Munich that there was "incontrovertible" evidence of Russian meddling in the 2016 presidential election.
He is the latest in a series of high-profile departures from the White House, with Mr Bolton becoming the third national security adviser in 14 months.
The move loads up the national security team with yet another figure who shares Mr Trump's penchant for exercising US power unilaterally after Mr Trump fired Secretary of State Rex Tillerson last week, also via Twitter, replacing him with former CIA Director Mike Pompeo.
Several other top-level staffers have left the administration in the past six weeks.
Earlier in March the director of the National Economic Council Gary Cohn resigned after he failed to stop Mr Trump from imposing import tariffs on steel and aluminum.
At the end of February, White House communications director Hope Hicks, one of Trump's longest-serving and most trusted aides, also resigned.
She was the fourth person to hold the post since Mr Trump became president.
White House staff secretary and senior advisor Rob Porter also resigned in early February following accusations of domestic abuse from two former wives.