Britain is hosting a two-day gathering of NATO leaders, marking the alliance's 70th anniversary.
The Prime Minister is expected to appeal for unity in NATO, amid increasing strains between member countries over the conflict in Syria.
The summit comes as tensions continue between French President Emmanuel Macron and Turkish President Recep Tayyip Erdogan over Turkey's recent incursion into northern Syria.
Mr Macron infuriated Ankara when he suggested NATO was suffering from "brain death" over the lack of co-ordination and leadership over Turkey's "crazy" attack on the Syrian Democratic Forces (SDF).
The group is largely headed up by Kurdish militia who helped the US to defeat the so-called Islamic State group (IS) before President Trump controversially withdrew troops from the region and left the Kurds to face Turkey’s cross-border offensive.
A Russian-brokered, conditional ceasefire has since been agreed.
The Turkish president hit back, claiming Mr Macron has "a sick and shallow understanding" of terrorism, suggesting he was the one who was "brain dead".
Alarm of further cracks in NATO relations spread further when Turkey purchased Russian air defence systems.
At a joint press conference with NATO Secretary General Jens Stoltenberg last week, Mr Macron called for "real dialogue" over NATO's future.
US President Donald Trump arrived in the UK on Monday, ahead of a formal reception for leaders this evening at Buckingham Palace, hosted by the Queen.
Mr Trump has previously voiced concerns over collective defence spending within the alliance.
The main talks take place on Wednesday at The Grove, a country house hotel near Watford, and are expected to address emerging threats in cyber and space - both now declared operational domains alongside sea, air and land.
Cover image: PA.