NATO leaders met Friday for a crucial summit in Warsaw to send a clear message to a resurgent Russia while trying to contain the fallout from Britain's dramatic divorce from the European Union.
Britain's future will dominate talks between US President Barack Obama, attending his last NATO summit, EU President Donald Tusk and European Commission chief Jean-Claude Juncker.
"This may be the most important moment for our transatlantic alliance since the end of the Cold War," Obama wrote in the Financial Times as the two-day summit got underway.
He said he was also "confident that the UK and the EU will be able to agree on an orderly transition to a new relationship".
Obama was due to meet British Prime Minister David Cameron, who is set to resign over the Brexit vote, at the summit.
NATO chief Jens Stoltenberg insisted on Friday that while the June 23 referendum result would inevitably change Britain's ties to the EU, "it will not change UK's leading position in NATO."
Obama was meanwhile being briefed on the shootings in Dallas, where five police officers were killed by snipers during protests against police shooting deaths of black Americans.
"The President has been updated on the shooting of police officers in Dallas. He asked his team to keep him updated on the situation as they get additional information," the White House said.
Security was tight across the Polish capital with police locking down roads to the venue in Poland's national stadium and helicopters flying overhead.
The uncertainty over Britain, a key nuclear-armed ally, comes as NATO prepares to endorse its biggest revamp since the end of the Cold War in response to Russia's 2014 intervention in Ukraine.
The summit centrepiece is a "Readiness Action Plan" to bolster NATO resources and readiness in the face of a Russia under President Vladimir Putin that the allies now see as more aggressive and dangerously unpredictable.
NATO leaders will approve rotating four battalions in eastern Europe -- in Poland, Estonia, Latvia and Lithuania, up to 4,000 troops in all, as a collective tripwire against fresh Russian adventurism.
Stoltenberg said this would "make clear that an attack on one ally will be met by forces from across the whole alliance."
The plan also includes a pledge to spend two percent of annual economic output on defence, ending years of cuts, and the creation of a 5,000-strong "Spearhead" force ready to deploy within days.
German Chancellor Angela Merkel said Thursday that while Russia's actions in Ukraine had eroded mutual trust, the alliance should balance "deterrence and dialogue" with Moscow.
Stoltenberg however sounded a conciliatory note on Russia, with the alliance due to hold fresh talks with Moscow just after the summit.
"Russia is our biggest neighbour and an integral party of European security so sustaining dialogue is essential," Stoltenberg said. "NATO does not want a new Cold War. The Cold War is history and should remain history."
Russia has said it will examine closely what is decided at the summit for how it affects its security.
Moscow bitterly opposes NATO's expansion into its Soviet-era satellites, which it sees as a threat to its own security.
It is even more strident in its opposition to the Ballistic Missile Defence system the United States is building and which the summit is due to declare has reached an initial operating level.
Washington says the shield is designed to counter missile threats from Iran or the Middle East but Russia says that once the system becomes fully operational in 2018, it will undercut its strategic nuclear deterrent.
Separately Russia warned Friday that the US deployment of an advanced missile defence system in South Korea would have "irreparable consequences", echoing warnings by China.
The NATO leaders will discuss the deteriorating situation in Afghanistan where Obama on Wednesday said he would keep 8,400 troops into next year to tackle the Taliban.
The summit meanwhile will also approve a NATO-EU cooperation accord, laying out how the alliance -- which includes 22 of the 28 EU member states -- can work with the EU.