U.S. President Donald Trump shakes hands with France's President Emmanuel Macron, ahead of the NATO summit in Watford, in London, Britain, December 3, 2019. Trump and Macron clashed over the future of NATO on Tuesday before a summit intended to celebrate the 70th anniversary of the Western military alliance. Ludovic Marin/Pool via REUTERS

U.S. President Donald Trump with France's President Emmanuel Macron during a meeting ahead of the NATO summit in Watford, in London, Britain, December 3, 2019. In sharp exchanges underlining discord in a transatlantic bloc hailed by backers as the most successful military pact in history, Trump demanded that Europe pay more for its collective defence and make concessions to U.S. interests on trade. REUTERS/Kevin Lamarque

U.S. President Donald Trump looks on as France's President Emmanuel Macron talks, during a meeting ahead of the NATO summit in Watford, in London, Britain, December 3, 2019. Macron, the French president, stood by comments he made last month describing NATO as suffering from a lack of strategic purpose akin to "brain death", and criticised fellow NATO member Turkey, which he accused of working with Islamic State proxies. Ludovic Marin/Pool via REUTERS

U.S. President Donald Trump reacts as he meets France's President Emmanuel Macron, ahead of the NATO summit, in Watford, in London, Britain, December 3, 2019. Washington and Paris have long argued over NATO's purpose - France opposed the 2003 Iraq war - but the new tensions will add to doubts over the alliance's future that have grown with Trump's ambivalence over U.S. commitments to defend Europe. Ludovic Marin/Pool via REUTERS

France's President Emmanuel Macron gestures during the meeting with U.S. President Donald Trump, ahead of the NATO summit in Watford, in London, Britain, December 3, 2019. Trump said Macron's criticism of NATO was "very, very nasty" and questioned whether the U.S. military should defend any countries that were "delinquent" on alliance targets for national military spending. Ludovic Marin/Pool via REUTERS

U.S. President Donald Trump meets with France's President Emmanuel Macron, ahead of the NATO summit in Watford, in London, Britain, December 3, 2019. All 29 member states have a target of spending 2% of their gross domestic product on defence and Trump has singled out Germany for falling short of that goal. But Macron stood by his criticism of NATO and said its real problem was a failure to forge a clear purpose since the end of the Cold War. Ludovic Marin/Pool via REUTERS