President Donald Trump has forced out his national security adviser John Bolton, with whom he had significant disagreements on Iran, Afghanistan and other global challenges.
President Trump tweeted that he told Mr Bolton on Monday night his services were no longer needed at the White House and Mr Bolton submitted his resignation on Tuesday morning.
President Bolton responded in a tweet of his own that he offered to resign on Monday "and President Trump said, 'Let's talk about it tomorrow'".
Reports have suggested that Mr Bolton's opposition to a possible meeting between Mr Trump and Iranian President Hassan Rouhani was a precipitating factor in the dismissal.
The adviser has also been sceptical about the President's whirlwind rapprochement with North Korea and his decision to pull US troops out of Syria last year.
Tensions have also risen between Mr Bolton and Secretary of State Mike Pompeo over how to manage the President's desire to negotiate with some of the world's most controversial leaders.
For example, just last week President Trump was considering bringing Taliban negotiators to Camp David to try to finalise a peace deal in Afghanistan.
However, Mr Bolton did strongly disagree with this notion, and it was finally scrapped.
Mr Bolton was named President Trump's third national security adviser in March 2018 after the departure of Army General HR McMaster, but was already a household name through his outspoken support for the Iraq War as the US ambassador to the UN under George W. Bush.
The President has said he will name Mr Bolton's replacement next week.
You will be able to hear in-depth analysis on this story from Dr Karin von Hippel and Professor Fawas Gerges on the next Sitrep.