John Bolton, President Donald Trump's former national security adviser, is back in the spotlight with the upcoming release of his memoir "The Room Where It Happened."

Bolton characterizes Trump as erratic and unfit, and makes a number of explosive claims about his handling of foreign policy and national security.

The Justice Department is currently suing Bolton to prevent the book from being released on its scheduled publication date of June 23 as Trump denounces it as full of "lies" and "made up stories."

Here's how Bolton went from being a mid-level bureaucrat in the Reagan administration to the most important person directing US national security policy, and now, one of Trump's biggest foes.

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John Bolton, President Donald Trump's former national security adviser, is back in the spotlight with the upcoming release of his new tell-all memoir "The Room Where It Happened," set to be published by Simon & Schuster on June 23.

The New York Times, Washington Post, and Wall Street Journal published details from the memoir on Wednesday based on prepublication copies they obtained of the book.

Bolton, who characterizes Trump as an erratic and unfit president, makes a number of explosive and damning claims about Trump's handling of foreign policy and national security matters in the book.

Bolton says that in addition to being un-interested and un-informed about several aspects of foreign policy, Trump went out of his way to appease and curry favor with foreign dictators and authoritarians like Russian President Vladimir Putin, North Korean leader Kim Jong-un, and Chinese President Xi Jinping.

As the Justice Department sues Bolton to prevent the book's public release, arguing that it contains classified information, Trump is publicly blasting the memoir as "a compilation of lies and made up stories, all intended to make me look bad."

Bolton's re-emergence in the public eye comes months after Trump abruptly announced in a September 2019 tweet he had fired Bolton because Trump "disagreed strongly with many of his suggestions."

Bolton, a graduate of Yale law school, began working on foreign policy and national security issues under the Reagan administration in the early 1980s, where he quickly developed a reputation as a defense hawk skeptical of the US bureaucracy and international institutions.

Over the years, Bolton has held some controversial stances, including advocating for preemptive military strikes on North Korea and Iran, regime change in the latter, and pushing Russia for harboring Edward Snowden.

As an undersecretary for arms control in the Bush administration, Bolton also pushed the now-discredited belief that Iraq was in possession of weapons of mass destruction which served as the justification for the US invasion of that country.

He later served as the US ambassador to the United Nations — an institution he routinely criticized as ineffectual — where he continued to push for sanctions on countries accused of possessing or developing nuclear weapons.

Bolton served as Trump's national security adviser for about 16 months, and pushed to fulfill his decades-long vision of the US engaging in a direct confrontation with Iran, according to a May 2019 profile of Bolton published in the New Yorker.

He then became a key figure in Trump's impeachment, as some of his subordinates in the National Security Council testified that Trump and his associates held up a $400 million military aid package to Ukraine in exchange for their government announcing investigations into former Vice President Joe Biden and his son, Hunter.

Bolton, however, frustrated Democrats but not appearing to testify before the Democratic-controlled House of Representatives. The Republican-controlled US Senate did not vote to call him as a witness.

Here's how Bolton went from being a mid-level bureaucrat in the Reagan administration to the most important person directing US national security policy and now one of Trump's foes.