For nearly four years, US President Donald Trump has publicly opposed NATO, demanding that its members pay more for the collective defense of Europe. Privately, Trump went even further, repeatedly talking about a complete withdrawal from the military alliance, the NYT reported, citing sources.

And he may have such a chance if re-elected.

Recent reports from former senior national security officials in the Trump administration have fueled growing unrest on Capitol Hill and across Europe. They confirm a scenario in which Trump - emboldened by re-election and potentially surrounded by an inexperienced national security team - could finally undermine or even end the US' NATO membership.

Former officials have warned that the move would be one of the largest global strategic shifts in generations and a major victory for Russian President Vladimir Putin.

While Trump has been known to show interest in the United States' withdrawal from the NATO since 2018, further evidence of his views has emerged ahead of the November elections.

This summer, Trump's former national security adviser John Bolton published a book and writes that the president has repeatedly stated his desire to withdraw from the alliance. Bolton suggested last month that Trump might even deliver an 'October surprise' shortly before the election by announcing his intention to withdraw from the alliance.

And in a book published this week, Michael S. Schmidt, a reporter for the New York Times, wrote that former Trump chief of staff, John F. Kelly, said in an interview that one of the most difficult challenges he faced with Trump was to prevent him from leaving NATO.

While the president regularly demands that other NATO members spend more on defense, he has not publicly threatened to leave the alliance.