NATO leaders have pledged their commitment to the 29-member alliance's mutual defense clause while naming Russia a security "threat" in a joint declaration issued following their 70th anniversary summit in London.
"We reaffirm the enduring transatlantic bond between Europe and North America...and our solemn commitment as enshrined in Article 5 of the Washington Treaty that an attack against one ally shall be considered an attack against us all," the December 4 statement said.
The document also said that "Russia's aggressive actions constitute a threat to Euro-Atlantic security" and mentioned the need to address the "opportunities and challenges" presented by "China's growing influence and international policies."
Following the debate triggered by French President Emmanuel Macron's comments about the "brain death" of NATO, the alliance's leaders agreed on a "forward-looking reflection process...drawing on relevant expertise, to further strengthen NATO's political dimension including consultation," the statement said.
U.S. President Donald Trump canceled a planned news conference after the summit. He had repeatedly engaged in lengthy question-and-answer sessions with reporters on the first day of the summit, apparently prompting his fellow leaders to mock him.
"When today's meetings are over, I will be heading back to Washington," Trump tweeted. "We won't be doing a press conference at the close of NATO because we did so many over the past two days. Safe travels to all!"
As the event drew to a close, Trump voiced irritation with Canada's "two-faced" prime minister, Justin Trudeau, who was caught on camera apparently making joking remarks about Trump's lengthy press appearances.
Trudeau later played down the incident at a final news conference and said he and the U.S. leader had enjoyed a great meeting.
"Last night, I made a reference to the fact there was an unscheduled press conference before my meeting with President Trump, I was happy to take part in it, but it was certainly notable," Trudeau told reporters.
On the second and final day of their summit, the NATO leaders held a three-hour meeting at a luxury hotel and golf resort outside London dedicated to finding common ground amid infighting over the role and future of the seven-decade-old Western military alliance.
Before chairing the meeting, Secretary-General Jens Stoltenberg described NATO as "the most successful alliance in history because we have been able to change again and again when the world is changing."
"NATO is agile, NATO is active, NATO is adapting," he added.
The first day of the two-day gathering included a reception with Queen Elizabeth II at Buckingham Palace, a visit to 10 Downing Street, the British prime minister's official residence, and separate meetings between NATO leaders.
The day was overshadowed by disagreements between allies over issues including Turkey's military operation in Syria as well as last month’s comments by Macron that NATO was "brain-dead" and that the U.S. commitment to the bloc was fading.
Hours after landing in London, Trump hit back at Macron, saying he had been "very disrespectful" in what he said were "nasty" comments.
However, the U.S. president was more conciliatory toward the French leader at their joint press encounter later in the day, saying Washington and Paris shared "many good ventures," while playing down what he called a "minor dispute" on trade.
Two key issues have loomed over preparations for the summit: defense spending and relations between Turkey and other member states such as France.
Before departing for the British capital, Turkish President Recep Tayyip Erdogan said he would oppose NATO's plan for the defense of Baltic countries if the alliance did not support Turkey over its fight against Kurdish groups in Syria it considers terrorists.
Other leaders have opposed such a move, not least because Kurdish-led forces drove fighters of the Islamic State extremist group out of eastern Syria with the help of a multinational coalition led by the United States.
"I am confident that we will be able to also find a solution to the issue related to updating the revised defense plans," Stoltenberg told journalists on December 4, adding that he discussed the issue with Erdogan the previous night.
"We are working on the issue as we speak," he added.
Macron also called for comprehensive talks with Russia, amid persistent tensions between Moscow and the West. "It's important to have a strategic dialogue with Russia," he said, adding that the alliance should do so "with its eyes wide open."
President Vladimir Putin had earlier hinted that Russia was open to cooperation with NATO on common threats, including international terrorism, but complained that its attempts to forge closer links had "practically been curtailed."
Trump, who has pressed hard for other NATO member states to meet the alliance's spending rules, discussed the matter with Stoltenberg in London.
"We are making real progress, most importantly on the burden sharing. And your leadership on defense spending is having a real impact," Stoltenberg said at the meeting.
He said European allies and Canada had added $130 billion to defense budgets since 2016 and that this number would increase to $400 billion by 2024.
"This is unprecedented, this is making NATO stronger and it shows that this alliance is adapting, responding when the world is changing," Stoltenberg said.
NATO estimates show there are now a total of nine countries, including the United States, meeting the target agreed by members to spend at least 2 percent of their gross domestic product on defense.
"You could make the case that they've been delinquent for 25-30 years," Trump said, adding that the figure of 2 percent "is a very low number."
Trump later hosted a lunch for what he called the "2 percenters" to honor the eight other countries that meet that standard on defense spending.
"Lunch is on me," Trump said as he sat with representatives of Britain, Bulgaria, Estonia, Greece, Latvia, Lithuania, Poland, and Romania.