NATO leaders were preparing Friday to kick off a summit that the Western military alliance is heralding as a landmark, but that Russia has warned will worsen tensions in Eastern Europe.
"NATO will once again send a very clear message that we are here to protect and defend all allies in a new and more challenging security environment," Stoltenberg said in the Polish capital Warsaw before the start of the two-day meeting. "This will be a landmark summit."
"If the past seven decades teach us anything, it is that we will prevail if we stay united, strong and true to our democratic values. I am confident we will," US President Barack Obama wrote in an op-ed for the Financial Times published Friday.
The summit comes as relations between NATO and Moscow have hit a post-Cold War low, due to Russia's annexation of the Ukrainian peninsula of Crimea in March 2014 and its support for separatists in eastern Ukraine.
Both sides have been flexing their military muscles in Eastern Europe ever since, accusing each other of sabre-rattling and provocations.
In Warsaw, NATO leaders are due to clear the way for the deployment of four battalions of up to 1,000 troops each to Estonia, Latvia, Lithuania and Poland, which feel threatened by Russia's actions in Ukraine. Support for a brigade in Romania is also foreseen.
"Together we have to ensure that everyone who, just for a moment, is tempted to apply the law of force will come to understand very quickly that it simply does not pay off," Polish President Andrzej Duda said at a pre-summit event in Warsaw on Friday.
Britain announced that it will deploy 500 troops in Estonia and 150 troops in Poland. The decision shows that "NATO stands together, that we are showing solidarity," Duda said.
But Moscow has repeatedly warned that an eastward NATO expansion threatens its national security.
"It is clear to every cognizant person that there will have to be a military response from our side," Russia's ambassador to NATO, Alexander Grushko, said in an interview with the daily Kommersant published on Thursday.
Stoltenberg said there would be no justification for a Russian reaction because NATO's actions are defensive and being taken in response to Moscow's actions in Ukraine. He said NATO does not want "a new Cold War" and will continue to seek dialogue with Russia.
Moscow is also likely to be irritated by progress made on NATO's missile defence system in Europe. Command and control over the system will be handed from the United States to NATO at the summit.
Other issues on the agenda include support for the US-led coalition fighting Islamic State extremists in Syria and Iraq; NATO support for Afghanistan; and cooperation with the European Union, notably in the Mediterranean Sea.
Britain's vote to leave the European Union will cast a shadow over the summit, although officials have been at pains to argue that the move will not affect the British NATO membership.
The country is the second-largest military force in the alliance after the United States and provides close to one fourth of the total defence spending among European NATO allies, Stoltenberg said.
"I'm absolutely certain that the UK will continue as a strong and committed ally in NATO," he said.
Britain's departure from the EU may even lead the country to step up its role in NATO, Duda predicted. But he also warned of risks posed by discontent in Scotland and Ireland after the vote, and the danger this could pose for Britain.
Poland has mobilized some 10,000 people to ensure security at the Warsaw summit. But unlike at earlier such meetings, no major protests are expected.