Donald Trump was once reported to have joked of John Bolton in an Oval Office meeting that his advisor had 'never seen a war he didn't like'.
And with a reputation as a hardline foreign policy hawk, Mr Bolton was known to have pressed his boss for a harder line on sensitive diplomatic issues like North Korea, Iran, Russia and Afghanistan.
But now a man who was considered one of the most powerful in the White House has suddenly gone.
The President tweeted: "I informed John Bolton last night that his services are no longer needed...I disagreed strongly with many of his suggestions, as did others in the Administration."
Mr Bolton is the third security chief to leave the White House, an unprecedented situation in US as Giampiero Gramaglia explained to SBS Italian.
