John Bolton served as Trump’s national security advisor during part of his first term, but has since become a vocal critic of the president.

Bolton served as Trump’s third national security advisor—following the brief stint of Mike Flynn and the stormy tenure of H.R. McMaster. Bolton succeeded McMaster on April 9, 2018 and remained national security advisor until he resigned on September 10, 2019. After leaving the Trump administration, he wrote a book about his tenure.