Trump's national security team is increasingly being populated by figures who share his penchant for exercising US power unilaterally.
It's the third national security advisers in 14 months.
U.S. President Donald Trump announced in a tweet he was ousting H.R. McMaster - who replaced Michael Flynn after a month - with John Bolton - an announcement Bolton says he was not expecting.
The long time polarizing figure in Washington foreign policy circles has advocated using military force against Iran and North Korea and has taken a hard line against Russia.
Trump's decision to replace McMaster was the latest high-level departure from the Trump administration.