WARSAW -- NATO leaders have endorsed a major new deployment of armed forces to Eastern Europe, a direct response to growing belligerence from Russia and the largest such move by the alliance since the end of the Cold War.

The decision came as heads of state gathered in the Polish capital July 8 for a two-day summit that U.S. President Barack Obama said “may be the most important moment for our transatlantic alliance" in 25 years.

Aside from Russia, the alliance faces a growing number challenges including Islamic State extremists, cyberattacks, and the influx of millions of people seeking refuge in Europe. Also looming in the background is Britain's vote last month to leave the European Union.

The leaders from the 28 members formally authorized four multinational battalions of up to 1,000 troops to be led by Canada, Germany, Britain, and the United States. They will be stationed in Poland and the three Baltic states.

NATO Secretary-General Jens Stoltenberg called the new deployments, which had been announced earlier, an appropriate deterrence against Russia.



"We have just taken decisions to deliver 21st century deterrence and defense in the face of 21st-century challenges," Stoltenberg told reporters.

Much of the summit’s focus is on Russia, which seized the Crimean Peninsula from Ukraine in March 2014 and backs separatists whose war with Kyiv’s forces has killed more than 9,300 people in Ukraine's east.

Obama reiterated that in a commentary published on the Financial Times website shortly before the summit began.

“Russia’s aggression against Ukraine threatens our vision of a Europe that is whole, free, and at peace,” Obama wrote.

He highlighted attacks that have been fueled by Islamic State militants’ extremist ideology, attacks that “slaughtered innocents in NATO countries, from Orlando to Paris to Brussels to Istanbul.” And he focused on Britain’s vote to leave the European Union, and conflicts “from Africa to Syria to Afghanistan” that have sent migrants to Europe.

“I believe that our nations must summon the political will, and make concrete commitments, to meet these urgent challenges,” wrote Obama, who also met with EU leaders. “In Warsaw, we must reaffirm our determination -- our duty under Article 5 of the North Atlantic Treaty -- to defend every NATO ally.”

WATCH: Ukraine Hopes For Help Against Russian "Escalation" -- Deputy PM

The treaty’s Article 5 is the most important component of the alliance, obligating all members to come to the aid of another member if it is attacked. The clause has been invoked only once in the alliance’s 67-year history: after the terrorist attacks on the United States on September 11, 2001.

Russia’s interference in Ukraine has increased concerns in Poland and Estonia, Latvia, and Lithuania, which were under Moscow’s thumb until the disintegration of the Soviet Union a quarter-century ago. All are now NATO members.

"We are witnessing the policy of aggression and notorious lack of respect for international law, internal sovereignty, and territorial integrity," the summit host, Polish President Andrzej Duda, said in opening remarks.

Further reflecting the unease many European nations are feeling toward Russia, the leaders of Finland and Sweden -- neither of whom are members -- were attending the summit for the first time. Governments in both countries have openly discussed the possibility of closer cooperation, or even outright membership, in the alliance, a possibility that prompted thinly veiled threats from Moscow.

The U.S.-led battalion comes on top of an additional armored U.S. brigade, which U.S. officials announced earlier this year would begin rotating into Eastern Europe on a regular basis. That brings the number of fully manned U.S. combat brigades with a presence in Europe to three. A brigade comprises about 4,200 to 4,500 troops.

Stoltenberg and other leaders also tried to offer a fig leaf to Moscow, saying alliance would "continue to seek meaningful and constructive dialogue" with Russia, which he called “an integral part of European security."

WATCH: U.S. President Barack Obama said the United States and Europe were united in supporting Ukraine and maintaining sanctions on Russia, ahead of the annual NATO summit in Warsaw. (Reuters)

​

"NATO does not seek confrontation.... The Cold War is history and should remain history,” he said.

The NATO-Russia Council, which was set up in the 1990s to address Russia’s misgivings about the alliance expanding eastward, is to meet next week for the second time this year. The council was suspended in 2014 following Russia’s seizure of Crimea.

French President Francois Hollande also sounded a conciliatory note toward Russia, saying it should not be considered a threat but rather a partner.

"NATO has no role at all to be saying what Europe's relations with Russia should be. For France, Russia is not an adversary, not a threat," Hollande said.

"Russia is a partner which, it is true, may sometimes, and we have seen that in Ukraine, use force which we have condemned when it annexed Crimea," he added.

Earlier, Duda took an even harder line, saying NATO must stand firm in the face of what he called Russian “blackmail and aggression.”

“Everyone who is tempted to apply the rule of force even for a moment” must be made to “understand quickly that is does not pay off,” Duda said.

Ben Rhodes, a top White House official, also reiterated the stern message intended for Russia, saying Moscow's continued aggression would provoke a NATO repsonse.



"What we are demonstrating is that if Russia continues this pattern of aggressive behavior, there will be a response and there will be a greater presence in Eastern Europe," said Rhodes, a deputy national security adviser.

Destabilization

In addition to military force, Western governments say President Vladimir Putin’s Russia has used cyberattacks, propaganda, and other methods in an effort to destabilize European countries and undermine Western unity.

In Moscow, Kremlin spokesman Dmitry Peskov said that it was “absurd to speak of a threat from Russia” and that Moscow hoped "common sense" would prevail at the summit.

“Russia was and is open to dialogue and interested in cooperation -- but only on a mutually beneficial basis and taking into account mutual interests,” Peskov said in a conference call with journalists on July 8.

In an interview in the newspaper Kommersant, Russia's ambassador to NATO, Aleksandr Grushko, said the alliance has a “confrontational agenda" and that Moscow would take countermeasures.

NATO leaders, however, have said Russia’s aggression in Ukraine was what led to the deploying of additional forces. They have also taken Moscow to task for potentially dangerous maneuvers in recent months such as jets buzzing U.S. warships.

Critics of the increased NATO deployments say they are too small to serve as a serious deterrent and may only increase Russia’s ire.

But former U.S. Secretary of State Madeleine Albright said that "Putin needs no provoking -- he is the provocateur.”

“We need to remember that Putin will be far less likely to engage in provocation if he sees a NATO that is unified, strong, and determined to push back against any aggressive move on his part,” Albright said at a discussion of experts held alongside the summit.

One thing that is not expected is substantial movement toward NATO membership for Ukraine or for Georgia. Those two countries’ aspirations join the alliance were a catalyst of a five-day war in 2008 during which Russian forces drove deep into the former Soviet republic.

Montenegro, however, is participating in the Warsaw Summit as an observer after signing a preliminary agreement in May. The Balkans nations is expected to formally join the alliance next year.

Beyond NATO, Obama said that “our alliance must do more on behalf of global security, especially on Europe’s southern flank. NATO should intensify its commitment to the campaign to destroy (IS) and do more to help the EU shut down criminal networks that are exploiting desperate migrants crossing the Mediterranean and Aegean seas.”

He said his decision to maintain 8,400 U.S. troops in Afghanistan though the end of his presidency “should encourage more allies and partners to affirm their commitment to the NATO mission to train Afghan forces.”



With reporting by AP, dpa, Interfax, The Financial Times, Reuters