Nato leaders are meeting near London, as tensions between members threaten to overshadow a summit marking the military alliance's 70th birthday.
The three-hour talks are expected to cover issues such as cyber-attacks and the strategic challenge posed by China.
In opening remarks, UK Prime Minister Boris Johnson reminded members of the alliance's principle of "one for all, and all for one".
Tuesday saw sharp exchanges between the US and French leaders over many topics.
The highly choreographed anniversary gathering, intended to show unity, has been unable to hide deep differences between member states, BBC defence correspondent Jonathan Beale reports.
Although the 29-member bloc's future is not in doubt, there are disagreements over Turkey's recent military action in northern Syria, the levels of military spending by members and recent comments by French President Emmanuel Macron that the alliance is "brain dead".
Despite the divisions, Mr Johnson described Nato as a "giant shield of solidarity" that "protects nearly a billion people", saying at the start of the meeting at a luxury resort in Watford: "As long as we stand together, no-one can hope to defeat us."
Nato Secretary General Jens Stoltenberg cited increased spending commitments on defence by European allies and Canada, saying: "Nato is the most successful alliance in history because we've changed as the world has changed".
On Tuesday, he said those nations had added $130bn (£100bn) to defence budgets since 2016 and that this number would increase to $400bn by 2024. US President Donald Trump has frequently and forcefully criticised how much other allies spend on defence.
How sharp were the exchanges?
The first day of the special anniversary summit saw tensions bubble to the surface, with Mr Trump and Mr Macron sparring over Nato's role, Turkey, and Islamic State group (IS) fighters during a news conference.
Relations between the two leaders were already strained amid a dispute over taxes and trade, and comments from the French president last month that the US commitment to the alliance was fading.
Mr Trump, who once called Nato "obsolete", had earlier hit back by saying Mr Macron had been "very disrespectful" by describing Nato as "brain dead", calling them "nasty" comments. Mr Macron said he stood by his remarks.
"You just watched his team's jaws drop to the floor," Mr Trudeau said to a group that included Mr Johnson and Mr Macron during a reception at Buckingham Palace, without mentioning the US president's name. None of them appeared to realise that they were being recorded.
What's the background to the tensions?
Nato, the North Atlantic Treaty Organisation, started out in 1949 with just 12 countries as members but having now expanded to a bloc of 29, it is increasingly difficult for the alliance to project a united front.
Apart from defence spending - a longstanding issue of concern for the US, which militarily dominates the group - relations between Turkey and other member states is the other key issue looming over this summit.
Before departing for London, Turkish President Recep Tayyip Erdogan said he would oppose a Nato defence plan for the Baltic region and Poland if the bloc did not support Turkey over its fight against Kurdish groups in Syria it considers terrorists.
But other leaders, including Mr Macron, have made clear they oppose such a move, not least because Kurdish-led forces were key allies in the US-led multinational coalition against IS in Syria.
Turkey's relations with other Nato members have been strained since it started a military offensive in Kurdish-controlled areas of northern Syria in October. That followed the unilateral decision by President Trump to pull US troops out of the region.
"When I look at Turkey, they now are fighting against those who fought with us shoulder to shoulder against [IS]," the French president said while sitting alongside Mr Trump, who faced heavy criticism for the withdrawal.
Mr Erdogan's decision to buy the S-400 Russian missile defence system further exacerbated tensions. Mr Trump said he was "looking at" imposing sanctions over the deal, while Mr Macron asked: "How is it possible to be a member of the alliance... and buy things from Russia?"
Later, Mr Macron attended a meeting in Downing Street with Mr Johnson as well as Mr Erdogan and German Chancellor Angela Merkel.
Meanwhile, Nato estimates for 2019 show there are now eight countries - in addition to the US - meeting the target agreed by all members to spend 2% or more of their gross domestic product (GDP, a measure of economic output) on defence.