Nato leaders have released a joint statement reaffirming their “enduring transatlantic bond” amid tensions on the alliance’s 70th birthday.
The three-hour talks near London resolved a disagreement with Turkey, which was blocking Nato’s updated defence plans in the Baltic region.
A row has also been building over a recording of Canadian leader Justin Trudeau talking about Donald Trump.
In response, US President Trump called Mr Trudeau “two-faced”.
President Trump also said he might cancel a planned press conference scheduled for after the summit, telling reporters: “We’ll go directly back. I think we’ve done plenty of news conferences.”
Mr Trump and French President Emmanuel Macron also had sharp exchanges over many topics on Tuesday.
What did the leaders’ joint statement say?
In the statement, Nato leaders said: “To stay secure we must look to the future together.”
It then acknowledged the “challenges” posed by China and Russia, and pledged to take “stronger action” against terrorism.
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, UK Prime Minister Boris Johnson – the host of the event – 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.President Trump and Mr Johnson held unscheduled bilateral talks and, ahead of Wednesday’s talks, Mr Trump tweeted they both had “talked about numerous subjects including Nato and trade”.
Meanwhile, a brief video posted on Twitter by Canada’s public broadcaster, CBC, showed Canadian Prime Minister Justin Trudeau apparently mocking Mr Trump’s impromptu news conference that lasted almost an hour on Tuesday.
“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 they were being recorded.
Mr Trump later called Mr Trudeau “two-faced” over the incident.
“With Trudeau, he’s a nice guy. I find him to be a nice guy, but the truth is, I called him out on the fact that he’s not paying 2% [of national GDP to defence] and I guess he’s not very happy about it,” he added.
“He’s not paying 2% and he should be paying 2%. Canada – they have money. Look, I’m representing the US and he should be paying more than he’s paying, he understands it… I can imagine he’s not that happy but that’s the way it is.”
Asked about the video, Mr Johnson told reporters: “It’s complete nonsense. I don’t know where that’s come from.”
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, made clear they opposed such a move, not least because Kurdish-led forces were key allies in the US-led multinational coalition against IS in Syria.
Mr Stoltenberg later told reporters that the Alliance had been able to resolve the disagreement with Turkey, allowing Nato’s plans to go ahead.
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.