Leaders of 28 nations belonging to the North Atlantic Treaty Organisation (Nato), the US-led military alliance in Europe, are meeting today in the Polish capital Warsaw for a two- day summit aimed at addressing the continent's security challenges.
Most of the talks will concentrate on plans to deploy Western troops to member states bordering Russia, in response to what Nato regards as a more aggressive Russian attitude towards Europe.
But US President Barack Obama and other alliance leaders will also use the summit to discuss the security implications stemming from Britain's decision to leave the European Union, as well as America's commitment to the continent.
Nato's biggest asset is that it is the only alliance in the world in whose defence the US remains legally pledged to go to war. That does not exempt Nato from tensions, especially over defence contributions. The Americans provide no less than 80 per cent of Nato assets, yet in terms of population and the combined size of their economies, the Europeans are bigger than the United States.
Mr Obama has frequently expressed his frustration at this lopsided arrangement.
Yet, stunned by Russia's military intervention in Ukraine in 2014, the Europeans are now devoting more money to their military. Most have pledged to boost spending to an agreed Nato-wide 2 per cent share of their gross domestic product.
Capabilities have also been boosted by Mr Obama's recent decision to increase his country's spending on military activities in Europe, to a total of US$3.4 billion (S$4.6 billion) yearly. Nato Secretary-General Jens Stoltenberg says that will fund "more troops in the eastern part of the alliance, as well as the pre-positioning of equipment, tanks and armoured vehicles".
At the summit, Nato countries are set to agree on the deployment of four multinational battalions - roughly 2,000 troops plus their equipment - to the Baltic states in northern Europe, plus one brigade which could ultimately have a similar number of personnel in Romania in the south-eastern part of the continent.
None of these deployments is permanent, to comply with agreements reached with Russia during the 1990s. But all will be "enduring", as Nato officials put it, meaning that alliance troops will be around Russia's borders on a semi-permanent basis, to send a political message that Nato is determined to defend its members. The former communist countries in central Europe, and particularly the Poles who are today's hosts, would have wanted Nato to do more, but they will be satisfied with getting this arrangement in place.
Still, alliance leaders will have plenty to fret about. The first source of worry is uncertainty in Europe about Mr Obama's Russia policy. The US leader has been robust in his criticism of Russian activities in Ukraine. But at the same time, US Secretary of State John Kerry appears to have given the Russians more or less a free hand in Syria, an arrangement that the Europeans fear might be interpreted in Moscow as a sign of American weakness.
Partly in order to dispel any doubts about US commitment to allies under Russian pressure, Mr Kerry arrives at today's summit fresh from a trip to Georgia, a former Soviet republic that fought a war with Russia in 2008. But the Europeans will want to hear further reassurances from Mr Obama himself. And Europe worries about the candidature of Mr Donald Trump, the first-ever Republican presidential contender to suggest that America should "reconsider Nato".
But the biggest worry is over Britain's decision to leave the EU. In theory, that does not affect Nato, where Britain remains a key player. But in practice, any tension between Nato and the EU, which is now inevitable as Britain remains in one organisation but not in the other, will not be good for European security. Member states will seek an assurance that Britain will not act as an obstacle to closer EU-Nato cooperation.
Still, the historic symbolism of today's summit will not be lost on anyone. For the Polish capital was the place where the old Soviet Union established the Warsaw Pact, the communist bloc's response to Nato.
Now, it is the alliance's turn to meet in precisely the same building where old Warsaw Pact commanders held their summits.