When NATO issues its communiqué on Saturday at the conclusion of its two-day summit in Warsaw, look for the word "unity" and its synonyms to appear early and often.
Unity is what the alliance wants to project in it showdowns with Russia to the east, and its even messier fight to the south, where Western-backed proxies battle both Islamic State and the regime of Bashar al-Assad in Syria. The refugee crisis – which has seen NATO members build fences along borders shared with other members of the alliance – is another challenge.
Unity is something that's suddenly in question as the alliance grapples with the meaning of Britain's shocking vote in favour of a Brexit from the European Union. The summit that began Friday – which was supposed to be a meeting devoted to countering the perceived threat from Russia (and symbolically held in the city where NATO's Cold War rival, the Warsaw Pact, was formed) – has become a "Brexit summit" devoted to figuring out how the alliance might be affected by the United Kingdom's looming departure from the EU.
"All the political leaders in Warsaw will be carefully following the messages from the British delegation, from Prime Minister [David] Cameron, because we'd like to know what kind of consequences NATO should take into account in its own policies from a possible Brexit," said Slawomir Debski, director of the Polish Institute of International Affairs, which is funded by the Polish government.
Mr. Debski said that while many in Europe were still hoping that a Brexit could somehow still be avoided – an increasingly distant possibility – "it would be crucial not to allow this process to influence the credibility of the alliance."
The alliance's rivals and enemies, of course, are hoping that's exactly what happens. Just two years after Russia's annexation of Crimea seemed to give it a fresh injection of relevance, the internal questions facing NATO include not only Brexit, but an assertion from Donald Trump, the presumptive Republican Party nominee for the U.S. presidency, that "here's the problem with NATO: It's obsolete."
On that point, Mr. Trump's argument lines up very close to Moscow's. Long before the outbreak of the conflict in Ukraine, Russian President Vladimir Putin was arguing that NATO's post-Cold War expansion into Eastern Europe – and later its support for the uprising against Kremlin ally Moammar Gadhafi in Libya – was pushing Russia and the West back into confrontation.
The chest-thumping continued on Friday in Warsaw, with NATO announcing the deployment of new combat battalions of about 1,000 soldiers into each of Estonia, Latvia, Lithuania and Poland. All four countries have an ingrained fear of Russian aggression bred during decades of Soviet occupation, and all were badly shaken by Russia's seizure of Crimea. The Kremlin is also accused of providing material support to separatist militias that have taken over large parts of eastern Ukraine, fuelling fighting that has killed 9,000 people in the past two years.
While the EU faces the prospect of shrinking, NATO is still looking to expand. The Warsaw gathering was expected to see the alliance officially welcome tiny Montenegro as its 29th member. Meanwhile, the leaders of Finland and Sweden – two countries that remained neutral throughout the Cold War – joined the main summit talks for the first time.
The announcements were met with a predictable growl from the Kremlin, which said the NATO deployments were "short-sighted" and motivated by "Russophobia."
"Russia is not looking [for an enemy] but it actually sees it happening," Mr. Putin's spokesman, Dmitry Peskov, told reporters in Moscow. "When NATO soldiers march along our border and NATO jets fly by, it's not us who is moving closer to the NATO borders."
But the Kremlin does see itself gaining from Brexit, which it hopes will fuel other Euroskeptic movements around the continent. Any crack in European unity is good news for Moscow, which is hoping Western countries will individually start lowering economic sanctions that were collectively imposed over the crisis in Ukraine.
Russian media have hailed Brexit as portending the end for the EU as a whole. Mr. Peskov compared it with the events that led to the breakup of the Soviet Union 25 years ago.
"If the entire EU were to fall apart, there would be a lot of policy-makers in Russia who would be happy," said Ian Kearns, director of the European Leadership Network, a London-based think tank. A less-united Europe, where Moscow could play individual countries off against each other, rather than having to deal with them as a single bloc "would be a very significant geopolitical shift in [Russia's] favour," Mr. Kearns said.
Mr. Cameron arrived in Warsaw with the message that Britain does not wish to see that happen. "Britain may be leaving the EU but we are not turning our back on Europe and we're not turning our back on European defence and security," he said shortly before the summit began.
To back up that message, Britain announced Friday that it would contribute 650 soldiers to the multinational battalion that will be deployed in Estonia. Britain is the alliance's second-largest military power, after the U.S., and also wields what is considered to be its second most-effective intelligence agency.
A Brexit potentially would see Britain cease sharing intelligence with its fellow EU partners, making intelligence co-operation within NATO even more critical.
Canada will contribute a similar number of troops and take the lead of the mission in neighbouring Latvia, while also contributing six CF-18 fighter jets and a frigate to the defence of the Baltic region.
Germany and the United States, respectively, will head the NATO forces arriving in Lithuania and Poland.
Despite a 1997 treaty between NATO and Russia that bars permanent new deployments in Eastern Europe, no timeline has been given for the withdrawal of the multinational battalions. Canada's Chief of the Defence Staff General Jonathan Vance called the mission "an open-ended commitment" that was "intended to be enduring."
Many in NATO contend that Russia fundamentally breached the 1997 pact when it sent troops into Crimea two years ago.
Mr. Kearns said he was worried the new deployments in Eastern Europe would contribute to the continuing cycle of "escalation and counterescalation" between NATO and Russia, one that has seen past NATO reinforcements met with large-scale Russian military exercises near the country's western borders.
But NATO's political leaders clearly feel they need to show resolve in the current environment.
"This may be the most important moment for our transatlantic alliance since the end of the Cold War," Barack Obama wrote in Britain's Financial Times newspaper Friday on the eve of his last summit as U.S. President. "Russia's aggression against Ukraine threatens our vision of a Europe that is whole, free and at peace. The vote in the U.K. to leave the EU raises significant questions about the future of European integration."