Luxembourg's Prime Minister Xavier Bettel and his husband Gauthier Destenay arrive for a reception at Downing Street hosted by Britian's Prime Minister Boris Johnson, ahead of the NATO summit in Watford, in London, Britain, December 3, 2019. Alastair Grant/Pool via REUTERS

France's President Emmanuel Macron, Britain's Prime Minister Boris Johnson, Turkish President Tayyip Erdogan and German Chancellor Angela Merkel pose as they meet at Downing Street ahead of the NATO summit in London, Britain, December 3, 2019. Murat Cetinmuhurdar/Presidential Press Office/Handout via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. TPX IMAGES OF THE DAY

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. Ludovic Marin/Pool via REUTERS

Britain's Queen Elizabeth receives the High Commissioner for Cyprus Andreas Kakouris during a private audience at Buckingham Palace in London, Britain December 3, 2019. Dominic Lipinski/Pool via REUTERS

Britain's Queen Elizabeth hosts a formal reception for the NATO leaders at Buckingham Palace in London, Britain December 3, 2019. Geoff Pugh/Pool via REUTERS

Iceland's Prime Minister Katrin Jakobsdottir leaves Downing Street after attending a reception hosted by Britian's Prime Minister Boris Johnson, ahead of the NATO summit in Watford, in London, Britain, December 3, 2019. Alastair Grant/Pool via REUTERS

London taxis drive past NATO and members' flags ahead of Nato Summit in London, Britain December 2, 2019.

Czech President Milos Zeman arrives at Buckingham Palace for a reception to mark 70 years of the NATO Alliance, hosted by Britain's Queen Elizabeth, in London, Britain, December 3, 2019. Dan Kitwood/Pool via REUTERS

Leaders of NATO alliance countries, and its secretary general Jens Stoltenberg, join Britain's Queen Elizabeth, Prince Charles, Prime Minister Boris Johnson, U.S. President Donald Trump, German Chancellor Angela Merkel, French President Emmanuel Macron and Canadian Prime Minister Justin Trudeau for a group picture as they gather for to mark 70 years of NATO Alliance during a reception at Buckingham Palace, London, Britain December 3, 2019. Yui Mok/Pool via REUTERS

Britain's Queen Elizabeth walks with U.S. first lady Melania Trump during a reception to mark 70 years of the NATO Alliance, hosted by Britain's Queen Elizabeth, at Buckingham Palace, in London, Britain, December 3, 2019. Yui Mok/Pool via REUTERS

U.S. President Donald Trump talks with France's President Emmanuel Macron, ahead of the NATO summit, in Watford, in London, Britain, December 3, 2019. Ludovic Marin/Pool via REUTERS

Poland's President Andrzej Duda and his wife Agata arrive for a reception at Downing Street hosted by Britian's Prime Minister Boris Johnson, ahead of the NATO summit in Watford, in London, Britain, December 3, 2019. Alastair Grant/Pool via REUTERS

Norway's Prime Minister Erna Solberg and her husband Sindre Finnes are greeted by Deputy Master of The Household Lt Col Anthony Charles Richards as they arrive at Buckingham Palace for a reception to mark 70 years of the NATO Alliance, hosted by Britain's Queen Elizabeth, in London, Britain, December 3, 2019. Dan Kitwood/Pool via REUTERS

U.S. President Donald Trump meets with NATO Secretary General Jens Stoltenberg, ahead of the NATO summit in Watford, in London, Britain, December 3, 2019.

German Chancellor Angela Merkel arrives for a reception in Downing Street, ahead of the NATO summit in Watford, in London, Britain, December 3, 2019.

U.S. President Donald Trump with his wife, first lady Melania Trump are seen together with Britain's Prince Charles and Camilla, Duchess of Cornwall during a reception at Buckingham Palace to mark 70 years of the NATO Alliance, hosted by Britain's Queen Elizabeth, in London, Britain, December 3, 2019. Geoff Pugh/Pool via REUTERS

Britain's Prince Charles meets Canadian Prime Minister Justin Trudeau at Clarence House, London, Britain, as NATO leaders gather to mark 70 years of the alliance, December 3, 2019. Victoria Jones/Pool via REUTERS

Turkey's President Tayyip Erdogan waves outside 10 Downing Street ahead of the NATO summit in London, Britain December 3, 2019. Daniel Leal-Olivas/Pool via REUTERS

Germany's Chancellor Angela Merkel arrives at Downing Street for talks with Britain's Prime Minister Boris Johnson, ahead of the NATO summit in Watford, in London, Britain, December 3, 2019.

U.S. President Donald Trump and U.S. first lady Melania Trump arrive at Clarence House to meet Britain's Prince Charles and Camilla, Duchess of Cornwall, ahead of the NATO alliance summit, in London, Britain December 3, 2019. Nicholas Kamm/Pool via REUTERS

Britain's Prince Charles meets Canadian Prime Minister Justin Trudeau at Clarence House, London, Britain, as NATO leaders gather to mark 70 years of the alliance, December 3, 2019. Victoria Jones/Pool via REUTERS

U.S. President Donald Trump looks at France's President Emmanuel Macron talk, during a meeting ahead of the NATO summit in Watford, in London, Britain, December 3, 2019.

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. Ludovic Marin/Pool via REUTERS

Dutch Prime Minister Mark Rutte arrives for a reception in Downing Street, ahead of the NATO summit in Watford, in London, Britain, December 3, 2019.

Staff members prepare a red carpet outside Downing Street ahead of arrival of NATO leaders, in London, Britain, December 3, 2019.

Britain's Queen Elizabeth receives the High Commissioner for Cyprus Andreas Kakouris and Kareen Farrell Kakouris during a private audience at Buckingham Palace in London, Britain December 3, 2019. Dominic Lipinski/Pool via REUTERS

London taxi drives past NATO and members' flags ahead of Nato Summit in London, Britain December 2, 2019.

France's President Emmanuel Macron looks on during a meeting with U.S. President Donald Trump (not pictured), ahead of the NATO summit in Watford, in London, Britain, December 3, 2019.