Members of the Royal Guard stand during U.S. President Joe Biden and first lady Jill Biden visit to Britain's Queen Elizabeth, at Windsor Castle in Windsor, Britain, June 13, 2021. Chris Jackson/Pool via REUTERS

Turkey's President Tayyip Erdogan fist bumps U.S. President Joe Biden during a plenary session at a NATO summit in Brussels, Belgium, June 14, 2021. Olivier Matthys/Pool via REUTERS.

U.S. President Joe Biden laughs while speaking with Britain's Prime Minister Boris Johnson during their meeting, ahead of the G7 summit, at Carbis Bay, Cornwall, Britain June 10, 2021.

U.S. President Joe Biden walks in front of Britain's Queen Elizabeth, at Windsor Castle in Windsor, Britain, June 13, 2021.

Britain's Queen Elizabeth and U.S. President Joe Biden stand in front of members of the Royal Guard, as they meet at Windsor Castle in Windsor, Britain, June 13, 2021.

U.S. President Joe Biden walks in front of members of the Royal Guard, at Windsor Castle in Windsor, Britain, June 13, 2021.

U.S. President Joe Biden says goodbye to first lady Jill Biden, as he departs Heathrow Airport on his way to Belgium, in London, Britain, June 13, 2021.

U.S. President Joe Biden delivers remarks to U.S. Air Force personnel and their families stationed at RAF Mildenhall, ahead of the G7 Summit, near Mildenhall, Britain June 9, 2021.

U.S. President Joe Biden inspects a Guard of Honour after arriving to meet Britain's Queen Elizabeth at Windsor Castle, in Windsor, Britain, June 13, 2021. Richard Pohle/Pool via REUTERS

NATO Secretary General Jens Stoltenberg welcomes U.S. President Joe Biden during the NATO summit at the Alliance's headquarters, in Brussels, Belgium, June 14, 2021. Patrick Semansky/Pool via REUTERS

France's President Emmanuel Macron talks with U.S. President Joe Biden before a plenary session at a NATO summit in Brussels, Belgium, June 14, 2021. Brendan Smialowski/Pool via REUTERS

France's President Emmanuel Macron talks with U.S. President Joe Biden, near Britain's Prime Minister Boris Johnson before a plenary session at a NATO summit in Brussels, Belgium, June 14, 2021. Brendan Smialowski/Pool via REUTERS

Women wave next to a sign welcoming President Joe Biden during his visit to the NATO summit in Brussels, Belgium, June 14, 2021.

U.S. President Joe Biden and NATO Secretary General Jens Stoltenberg bump elbows as they meet during a NATO summit, at the Alliance's headquarters in Brussels, Belgium, June 14, 2021. Stephanie Lecocq/Pool via REUTERS

Britain's Queen Elizabeth stands with U.S. President Joe Biden and first lady Jill Biden in the Grand Corridor during their visit at Windsor Castle, in Windsor, Britain, June 13, 2021. Steve Parsons/Pool via REUTERS

U.S. President Joe Biden gestures as he boards a plane at Heathrow Airport on his way to Belgium, in London, Britain, June 13, 2021.

U.S.President Joe Biden and France's President Emmanuel Macron walk along the boardwalk during the G7 summit in Carbis Bay, Cornwall, Britain, June 11, 2021.

U.S.President Joe Biden and first lady Jill Biden stand next to Britain's Queen Elizabeth as they meet at Windsor Castle, in Windsor, Britain, June 13, 2021. Matt Dunham/Pool via REUTERS

U.S. President Joe Biden speaks with Britain's Prime Minister Boris Johnson during a plenary session at a NATO summit in Brussels, Belgium, June 14, 2021. Olivier Matthys/Pool via REUTERS

U.S. President Joe Biden attends a meeting with NATO Secretary General Jens Stoltenberg during a NATO summit, at the Alliance's headquarters in Brussels, Belgium, June 14, 2021. Stephanie Lecocq/Pool via REUTERS

U.S. President Joe Biden walks next to Britain's Queen Elizabeth, at Windsor Castle in Windsor, Britain, June 13, 2021. Chris Jackson/Pool via REUTERS

Belgium's Prime Minister Alexander De Croo, Croatia's President Zoran Milanovic, Poland's President Andrzej Duda and U.S. President Joe Biden pose for a family photo during the NATO summit at the Alliance's headquarters, in Brussels, Belgium June 14, 2021. Francois Walschaerts/Pool via REUTERS