Signing the joint declaration, European Council President Donald Tusk said “the EU and NATO face the same threats.”
He said the the EU and NATO will deepen cooperation to counter both physical and hybrid attacks.
“Our critical infrastructure could be attacked, our banking system could be hacked, or our citizens could be exposed to disinformation campaigns,” Tusk warned.
Heads of state are arriving at the venue for the Warsaw summit. British Prime Minister David Cameron said that his country, while it may leave the EU, will not turn its back on European defense.
U.S. President Barack Obama took advantage of his visit to Warsaw to voice concerns about a controversial new law allowing the Polish government to appoint the judges of its choosing to the country's constitutional court.
Critics say the changes undermine democratic standards and are part of the government's broader effort to seize more control over state institution.
Obama told reporters in Warsaw he had raised the issue with Polish President Andrzej Duda on the sidelines of the NATO summit.
"I insisted that we are very respectful of Poland's sovereignty and I recognize that parliament is working on legislation to take some important steps, but more work needs to be done," he said.
The European Commission has already given Poland an official warning and said the country faced sanctions unless it repealed the law.
NATO summit launches in Warsaw with a tribute to all soldiers in NATO member states, including those fallen in the line of duty.
NATO chief Jens Stoltenberg said the alliance would take "important decisions that will shape security for many years to come" during the summit.
NATO's deputy assistant secretary general for political affairs, James Appathurai, said supporting Afghanistan remains a "vital investment" for the alliance.
Ukrainian Deputy Prime Minister Ivanna Klympush-Tsintsadze told RFE/RL in Warsaw that her country hopes bilateral talks with NATO members at the summit will provide it with lethal and non-lethal aid to combat Russian "escalation" of the conflict in eastern Ukraine.
Here is the latest wrap from our news desk:
NATO leaders vowed on July 8 to strengthen the alliance’s defenses near its borders with Russia and address an array of challenges ranging from Moscow's assertive actions, Islamic State extremists, cyberattacks, and conflicts that have prompted millions of people to seek refuge in Europe.
With Britain's vote to leave the European Union casting a shadow on Western unity, heads of state are meeting in Warsaw for a two-day NATO summit that U.S. President Barack Obama said “may be the most important moment for our transatlantic alliance since the end of the Cold War.”
"Today, we come together at a time of increasing uncertainty," NATO Secretary-General Jens Stoltenberg said, opening the summit and thanking soldiers who stood at attention for protecting "nearly 1 billion people" in the alliance's 28 member-states in Europe and North America.
"We will take important decisions to strengthen our deterrence and defense to protect our countries from attacks from any direction," Stoltenberg said. "We will agree to enhance our forward presence here in Poland as well as in Estonia, Latvia, and Lithuania. We will also strengthen our presence in the southeastern part of our alliance. and we will step up our cyber and ballistic missile defenses."
Much of the 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 eastern Donbas region.
“Russia’s aggression against Ukraine threatens our vision of a Europe that is whole, free, and at peace,” Obama wrote in a commentary published on the Financial Times website shortly before the summit began.
He also highlighted IS attacks that have “slaughtered innocents in NATO countries, from Orlando to Paris to Brussels to Istanbul,” the ‘Brexit’ vote, 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.”
Russia’s interference in Ukraine has increased concerns in eastern NATO nations including 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.
"We are witnessing the policy of aggression and notorious lack of respect for international law, internal sovereignty, and territorial integrity," the summigt host, POlish Prwesident Andrzej Duda, said in opening remarks.
The leaders of Finland and Sweden are sitting beside their NATO counterparts at an alliance summit for the first time, reflecting growing cooperation amid an increase in Russian military activity near their borders.
Stoltenberg waid that the alliance has "come a long way" since its last summit in September 2014, when the West was still stunned by Russia's actions. "We have made NATO faster, stronger, and more ready. Today we will take the next steps."
The NATO chief said the allies will agree to the deployment of four multinational battalions of up to 1,000 troops led by Canada, Germany, Britain, and the United States. They are to be deployed in Poland and the three Baltic states on a rotating basis – part of what he called “the greatest increase of our collective defense since the end of the Cold War.”
"As the Alliance prepares to enhance our forward presence in eastern Europe. I can announce that the U.S. will be the lead nation for the NATO presence here in Poland. That means the U.S. will deploy a battalion, roughly 1,000 American soldiers, here in Poland on a rotational basis to serve shoulder to shoulder with Polish soldiers," Obama said after meeting with Polish President Andrzej Duda.
"In addition, when a new U.S. armored brigade begins rotating through Europe next year, its headquarters will be here in Poland," Obama said.
And Stoltenberg said NATO will also “transform a Romanian brigade into a multinational brigade to strengthen our posture in the southeastern part of the alliance.”
At the same time, Stoltenberg stressed that NATO will "continue to seek meaningful and constructive dialogue" with Russia, which he called “an integral part of European security."
"NATO does not seek confrontation.... The Cold War is history and should remain history."
Stoltenberg 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 to end the war in eastern Ukraine.
Without naming Russia
At the experts’ forum, 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 is does not pay off,” Duda said.
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.
Stoltenberg and EU leaders signed a deal that the NATO chief said gave "new substance, new impetus" to the EU-NATO partnership in facing challenges including hybrid warfare and cyberattacks.
Putin’s spokesman said that it is “absurd to speak of a 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 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 rejects these charges, saying that Russia’s aggression in Ukraine has forced a response and taking Moscow to task over potentially dangerous military moves such as jets buzzing U.S. warships.
Critics of the NATO moves say the planned deployments are not large enough to serve as a serious deterrent to Russia and may only increase its 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 the experts’ forum.
Obama said NATO and the EU must “increase our support for Ukraine as it defends its sovereignty and territorial integrity.”
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.
Montenegro, however, is participating in the Warsaw Summit as an observer after signing an Accession Protocol with NATO in May, and is expected to 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.”
And here is our very own Brian Whitwore, aka The Power Vertical, blogging from the summit: