US President Donald Trump has said he has fired his national security adviser John Bolton.
Mr Bolton - a veteran Washington figure - was the third person to hold the senior role in the Trump administration, taking over last year following the departure of HR McMaster.
Michael Flynn held the role for less than a month in early 2017 before he resigned amid a controversy over his contacts with the Russian ambassador to the US.
Bolton had previously served as a US ambassador to the UN under George W Bush, and earlier held several roles under the Reagan administration.
He's widely considered a foreign policy hawk, and is known for his strong support for regime change in Iran.
President Trump announced Mr Bolton had resigned after being told "his services are no longer needed at the White House".
The US president added he "disagreed strongly" with many of Bolton's suggestions.
In a tweet, however, Mr Bolton seemed to suggest he had been expecting a further conversation with President Trump:
I offered to resign last night and President Trump said, "Let's talk about it tomorrow."
— John Bolton (@AmbJohnBolton) September 10, 2019
The decision comes amid a number of high-profile foreign relations tensions involving the Trump administration and several other countries.
Relations with Iran have worsened over the past year, following the US withdrawal from the Iran nuclear deal.
There have been several incidents involving the two countries recently, including the shooting down of a US drone in the Strait of Hormuz in June.
Over the weekend, Donald Trump also said he had cancelled a 'secret meeting' with the Taliban in the US - an apparent collapse of a potential agreement between the US, the Taliban and the Afghan government after months of negotiations.