Nato leaders are gearing up for a long-term stand-off with Russia, ordering multinational troops to Poland and the three Baltic states as Moscow moves forward with its own plans to station two new divisions along its western borders.
The alliance's secretary-general Jens Stoltenberg said that on the first day of a landmark two-day summit, US President Barack Obama and leaders of the 27 other Nato countries also declared the initial building blocks of a ballistic missile defence system operationally capable.
The countries recognised cyberspace as a domain for alliance operations, committed to boosting their civil preparedness, and renewed a pledge to spend a minimum of 2% of their national incomes on defence.
Mr Stoltenberg said: "We have just taken decisions to deliver 21st century deterrence and defence in the face of 21st century challenges.
He said deployment of the new Nato units to Poland, Estonia, Latvia and Lithuania on a rotational basis would start next year, with no end date.
"It's an open-ended commitment and will last as long as necessary," he said. "And it is a new reality because we didn't have that kind of presence in the eastern part of the alliance before."
He announced plans as well for an enhanced Nato presence in the Black Sea region, where Russia has also reasserted its influence, with creation of a multinational brigade under Romanian and Bulgarian command.
Polish President Andrzej Duda, the summit's official host, warned that Western democratic values are being undermined by a "notorious lack of respect for international law" as well as terrorism and high-tech warfare, and said Nato needs a coherent strategy to address those problems.
After arriving in Warsaw, Mr Obama announced his decision to send an additional 1,000 US troops to Poland as part of the Nato effort to reinforce its presence on the alliance's frontiers near Russia.
Following bilateral talks, Mr Duda thanked Mr Obama, saying Poles "are grateful for the good will, for understanding that security is where the world's strongest army is, and that army is the US army".
In a column published in the Financial Times, Mr Obama, who is making what is expected to be his last trip to Europe as president, called on Nato to stand firm against Russia, terrorism and other challenges, and to "summon the political will, and make concrete commitments" to strengthen European cooperation after the UK voted to leave the European Union on June 23.
Creation of the new Nato units is vigorously opposed by the Kremlin. It follows a raft of other decisions taken during the last 22 months to increase Nato's ability to face Russia and other new security challenges, including tripling the size of the alliance Response Force to 40,000 and formation of a highly nimble Spearhead Force that can start to move within days.
As Mr Obama and the other heads of state and government were gathering in the Polish capital, a spokesman for Russian President Vladimir Putin said Moscow is willing to cooperate with Nato, even though he said it acts towards Russia like an enemy.
Russia "has always been open for dialogue" with Nato, especially to combat what it sees as a "genuine threat" - terrorism, spokesman Dmitry Peskov said.
"Russia is not looking (for an enemy) but it actually sees it happening," Mr Peskov told reporters in Moscow. "When Nato soldiers march along our border and Nato jets fly by, it is not us who are moving closer to the Nato borders."
But German defence minister Ursula von der Leyen, whose nation will furnish the core components of the new battalion going to Lithuania, called it an appropriate measure to counter what she called a "completely unpredictable and aggressive Russia".
Ms Von der Leyen said Poland and the Baltic states want greater protection because Russia's 2014 annexation of Crimea from Ukraine has proven Moscow "doesn't respect borders". The minister told German broadcaster ARD that Nato must maintain a dialogue with Russia, but from a "position of strength".
"It's important that Nato deploys with such strength that it's clear nobody can see an advantage in attacking this military alliance," she said.
Mr Stoltenberg said Canada will furnish the framework components for the battalion to be sent to Latvia, and that Britain will do the same for Estonia. Nato officials have estimated the four battalions will have a total strength of about 4,000 soldiers.
"Nato has responded with speed and determination," Mr Stoltenberg said. He added that the new units, composed of troops from different alliance members, will make clear to Russia and anyone else "that an attack on one ally would be considered an attack on the whole alliance".
Earlier, the secretary-general said keeping members of Nato safe also means supporting partner nations in the Middle East and North Africa menaced by extremist violence.
"It's not enough to keep our defences strong; we must help to make our partners stronger," Mr Stoltenberg told a pre-summit meeting of defence and security experts.
"Training local forces is often our best weapon against violent extremism," he said.
Also on the summit programme is increased assistance for Iraq's military, extension of the West's financial commitment to the Afghan military and police, aid for Tunisia, and getting Nato more involved in the campaign against the Islamic State group by authorising use of surveillance planes to assist the US-led coalition that is fighting the militants.
Warsaw may have become the world's most highly secured city during the summit, Nato's first since September 2014. The meeting is taking place after a series of recent extremist attacks around the globe.
Helicopters hovered on Friday above the National Stadium, the meeting's venue, while 6,000 police officers, backed up by soldiers, gendarmes, firefighters and other security officials, helped patrol the city of 1.7 million.
After the deliberations, Nato leaders dined in the same room of the Polish presidential palace where the now-defunct Warsaw Pact military alliance formed by the Soviet Union was created in 1955.