Trump 'much more serious' about leaving NATO than many realized: new book
President Donald Trump and Russian President Vladimir Putin meet in Helsinki (Screen cap).

Former President Donald Trump was reportedly far more serious about withdrawing the United States from the North Atlantic Treaty Organization than many realized.

Officials were concerned about Trump's behavior almost as soon as he took office, and one senior White House official told authors Peter Baker and Susan Glasser that the former president engaged in a sustained effort to withdraw from the treaty, according to excerpts from a new book published by CNN.

"[It was] much more serious than people realized," the former official said.

A top U.S. intelligence official told Baker and Glasser that Trump's behavior at the 2018 summit with Vladimir Putin in Helsinki, Finland, set off alarm bells about the former president's motives, after he sided with the Russian president against U.S. intelligence agencies over foreign interference in the 2016 election.

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"I never could come to a conclusion," said Dan Coats, the then-director of national intelligence. "It raised the question in everybody's mind: What does Putin have on him that causes him to do something that undermines his credibility?"

Former national security adviser John Bolton has said he had to repeatedly talk Trump out of withdrawing from NATO, and he has speculated that Trump would have left the military alliance had he won a second term.