US President Donald Trump and his 28 NATO counterparts meet in London on Tuesday to celebrate the 70th anniversary of the founding of the alliance, but their summit is overshadowed by disputes over the purpose and strategic direction of NATO.
The recent events in northeast Syria have sparked a controversy over NATO, with French President Emmanuel Macron criticizing some leaders of the alliance's member states, whom he described as "clinical death". Others, particularly German Chancellor Angela Merkel, distanced themselves from his comments.
Last Friday, Turkish President Recep Tayyip Erdogan described his French counterpart as suffering from "clinical death" rather than NATO.
In October, the United States withdrew troops from northeastern Syria without consulting NATO allies, paving the way for Turkey, an ally.
Also in NATO, to attack a Western-allied Kurdish militia that helped defeat the Islamic State in the region.
Ankara considers Kurdish fighters linked to the PKK, which is classified as a "terrorist organization."
Several other issues have led to tension within the alliance, notably Trump's continued calls for other countries - notably Germany - to increase defense spending, while questioning the value of the alliance.
Washington is also critical of Turkey's position - which has bought the Russian-made S-400 missile system - which it sees as a threat to NATO, especially Washington, which has a military partnership with Ankara.
The meetings are due to start on Tuesday with a breakfast meeting between Trump, who arrived in London yesterday evening, and European officials, and later in the day will hold talks with Macron.
On the sidelines of NATO meetings, a meeting will be held to discuss the Syrian situation between Erdogan, Macron, Merkel and British Prime Minister Boris Johnson.
On the same evening, NATO leaders will attend a reception with Queen Elizabeth II at Buckingham Palace, followed by a visit to 10 Downing Street, the prime minister's official residence.