NATO Holds Crucial Summit Amid Challenges From Russia, IS, 'Brexit'

European Council President Donald Tusk (left to right), U.S. President Barack Obama, and European Commission President Jean-Claude Juncker deliver remarks to reporters after their meeting at the NATO summit in Warsaw on July 8.

WARSAW -- Western leaders gathering for a NATO summit say the alliance is determined to meet challenges posed by an aggressive Russia, Islamic State (IS) extremists, Britain's vote to leave the European Union, and conflicts that have prompted millions of people to seek refuge in Europe.

In a commentary published on the Financial Times website on July 8, hours before the start of the two-day summit in Warsaw that will be the last one he attends as president, U.S. President Barack Obama said this "may be the most important moment for our transatlantic alliance since the end of the Cold War."

NATO will send a signal of deterrence to a pugnacious Russia and demonstrate unity as the West also grapples with deadly attacks by IS militants, an influx of migrants into Europe, the divisive "Brexit" vote, and a resurgent Taliban in Afghanistan.

Much of the focus will be on Russia, which angered the West and upset the post-World War II order in 2014 by seizing the Crimean Peninsula from Ukraine and backing separatists whose war with Kyiv's forces has killed more than 9,300 people in the eastern Donbas region.

"Russia's aggression against Ukraine threatens our vision of a Europe that is whole, free, and at peace," wrote Obama, who also highlighted IS attacks that have "slaughtered innocents in NATO countries, from Orlando to Paris to Brussels to Istanbul," the British vote, and conflicts "from Africa to Syria to Afghanistan" that have sent waves of 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 will also meet with EU leaders including British Prime Minister David Cameron, who is stepping down to make way for Britain's exit from the bloc.

'Special Relationship' Will Endure

Obama said that the "special relationship between the U.S. and the U.K. will endure" and expressed confidence that Britain and the EU will "agree on an orderly transition to a new relationship."

"Moreover, while 'Brexit' creates some uncertainty, our shared prosperity will continue to rest on the rock-solid foundation of NATO," Obama wrote.

Russia's interference in Ukraine has increased concerns in eastern NATO members including summit host Poland and the Baltic states of Estonia, Latvia, and Lithuania, which were under Moscow's thumb until the collapse of communism and the disintegration of the Soviet Union a quarter-century ago.

"In Warsaw, we must reaffirm our determination -- our duty under Article 5 of the North Atlantic Treaty -- to defend every NATO ally," Obama wrote in the FT. "We need to bolster the defense of our allies in Central and Eastern Europe, strengthen deterrence, and boost our resilience against new threats, including cyberattacks."

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)

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Obama Pledges Ukraine Support, Continued Sanctions On Russia

Speaking at an experts forum just before the formal start of the summit, NATO Secretary-General Jens Stoltenberg said the allies will agree in Warsaw to the deployment of four multinational battalions of up to 1,000 troops led by Canada, Germany, Britain, and the United States. Each is to be deployed in Poland and the three Baltic states on a rotating basis -- part of what Stoltenberg called the biggest security upgrade since the Cold War.

He also said NATO will "transform a Romanian brigade into a multinational brigade to strengthen our posture in the southeastern part of the alliance."

The alliance "will take decisions to further strengthen our collective defense and deterrence," he added.

At the same time, Stoltenberg stressed that NATO will "continue to seek meaningful and constructive dialogue" with Russia, which he called the alliance's "biggest neighbor and an integral part of European security."

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

He cited the creation in the 1990s of the NATO-Russia Council, which is to meet next week for the second time this year after a hiatus following Russia’s seizure of Crimea.

Obama said that "even as our nations remain open to a more constructive relationship with Russia, we should agree that sanctions on Russia must remain in place until Moscow fully implements its obligations" under the Minsk agreements, a 2015 accord aimed at ending the war in eastern Ukraine and resolve the status of separatist-held parts of the Donetsk and Luhansk regions.

Standing Up To Russia

At the experts forum just ahead of the formal start of the summit, Polish President Andrzej Duda said 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 it does not pay off," Duda said.

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

Putin's spokesman said that it was absurd for NATO to talk of any threat from Russia, and that Moscow hopes "common sense" will 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," Dmitry Peskov said in a conference call with journalists on July 8.

NATO's moves to bolster its defenses have angered Moscow, which has long accused the alliance of stoking hostilities with its eastward expansion over the past two decades.

In an interview published in the Russian newspaper Kommersant, Russia's ambassador to NATO, Aleksandr Grushko, accused the alliance of having a "confrontational agenda" and warned that Russia would take countermeasures.

NATO rejects these charges, saying that Russia's aggression in Ukraine has forced a response and criticizing Moscow over potentially dangerous military moves such as jets buzzing U.S. warships.

"We need to deepen security cooperation between NATO and the EU and increase our support for Ukraine as it defends its sovereignty and territorial integrity," Obama said.

One thing that is not expected in Warsaw is substantial movement toward NATO membership for Ukraine or for Georgia, whose aspirations to join the alliance were a catalyst of a five-day war in 2008 during which Russian forces drove deep into the former Soviet republic. Moscow recognizes the breakaway Georgian regions of Abkhazia and South Ossetia as sovereign states and has military forces based in both.

Montenegro, however, will participate in the Warsaw summit as an observer after signing an Accession Protocol with NATO in May, and is expected to join the alliance sometime next year.

Beyond NATO, Obama also 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 more than 8,400 U.S. troops in Afghanistan though the end of his presidency in January "should encourage more allies and partners to affirm their commitment to the NATO mission to train Afghan forces."

Obama's words about challenges echoed Stoltenberg, who said on July 7 that the alliance has reached a "defining moment for our security."

"The world is a more dangerous place than just a few years ago," Stoltenberg said in Warsaw. "NATO is responding with speed and with determination."

With reporting by the Financial Times, Reuters, AP, and dpa