When President Obama touches down Friday in Poland, he'll land on a continent roiled by the British vote to leave the European Union, but he is there to confront the critical juncture facing another Western institution: NATO.
The 28-nation military alliance is contending with a confluence of challenges, including Russian incursions in the east, attacks by Muslim extremists on European capitals and the threat of cyberwarfare. Together, they are testing whether NATO is capable of reshaping itself for 21st century battles, or at risk of becoming a Cold War-era relic.
Even the Republican seeking to succeed Obama, Donald Trump, has openly questioned NATO's relevance and capacity to serve U.S. interests.
"There hasn't been another inflection point like this for the alliance since the fall of the Berlin Wall and the dissolution of the Soviet Union in '89 to 1991," said Ambassador Doug E. Lute, the U.S. permanent representative to NATO.
At his fifth NATO summit, Obama can seize on his final opportunity to shape the institution as he did during the leaders' last meeting in Wales two years ago, when the rise of Islamic State and Russia's annexation of Crimea led the alliance to recommit to what Obama said were "the will, the resources and the capabilities" to fulfill its mission.
The issues have presented new challenges but also fresh purpose for the North Atlantic Treaty Organization, which will add a 29th member, Montenegro, at the summit.
Although the territorial advances by Islamic State that so concerned leaders at the 2014 summit have been turned back, the ongoing Syrian civil war as well as the terrorist network's demonstrated ability to strike beyond the Middle East, into European capitals directly and perhaps inspiring attacks like the mass shooting in Orlando, Fla., underscores the threat to NATO's southeastern front.
To the east, Russia continues to be a provocative force in Ukraine as well as for the Baltic and Black seas countries, leading NATO to undertake a rotation of four battalions in Eastern Europe.
Obama spoke Wednesday with Russian President Vladimir Putin and urged him to end increased fighting in eastern Ukraine and to make progress on the agreement reached in 2014 in Belarus to halt the war in Ukraine, according to a White House summary of their call.
The U.S. will ask NATO members to commit additional resources along the alliance's eastern boundaries "as we're reassuring those allies and making clear to Russia that we will not tolerate any type of aggression or intervention within NATO's borders," said Ben Rhodes, White House deputy national security advisor.
To the south, NATO partners are also grappling with a flow of migrants across the Mediterranean Sea while standing ready to support a fledgling government in Libya.
And the alliance has moved into new terrain with a dedicated focus on cyberthreats, now considered on the same level as threats by air, sea or land. Member nations are trying to decide how to cooperate on cybersecurity and what might constitute a cyberattack that would trigger Article 5 of NATO's treaty, which stipulates that an attack on one member nation is an attack on all.
"There is a lot of angst about offensive cyber, but active defense requires some offensive actions," retired Gen. Philip Breedlove, the former supreme allied commander of NATO, said at a recent summit hosted by the Center for Strategic and International Studies. "So there are policy decisions that need to be taken."
To that end, NATO is establishing a high-level intelligence post as part of its reorientation toward fighting modern, global threats. On Thursday, two top U.S. officials — James Clapper, the director of national intelligence, and Marcel Lettre, the undersecretary of Defense for intelligence — called for the post to be filled by November.
What will probably be Obama's final visit to Europe as president also bears the scars of the one that preceded it. In April, he stood beside British Prime Minister David Cameron to make a vigorous case for a vote by Britons to remain in the European Union. Days later in Germany, Obama, an avowed internationalist, urged the broader European community not to turn its back on its unique experiment in multilateral governance.
Obama and EU leaders will meet Friday for special sessions devoted to managing the fallout from Britain's vote, which has emboldened nationalist movements elsewhere. Part of Obama's challenge will be working to ensure that leaders' understandable preoccupation with the European Union does not distract from NATO's security goals.
Whether the European Union preserves its economic sanctions against Russia — a policy that Britain championed — will be one crucial test.
"It's impossible to overlook what a major blow [Brexit] is … to 60 years-plus of American policy promoting European integration and trans-Atlantic cooperation," said Jeffrey Rathke, a senior fellow at the Center for Strategic and International Studies and former deputy director of the NATO secretary general's private office.
"How the United States can shore up its European partners will be an important outcome, even if it's not directly on the NATO agenda."
The president will hail increased defense spending by NATO partners, according to the White House, which Obama has called for.
"For the first time in really about two decades, non-U.S. defense spending among NATO allies is on the increase," Lute said. "So that I think gives us confidence that while we have a long way to go, we've turned the corner and we're moving in the right direction."
In addition to pressure from Russia in Eastern Europe and the fight against Islamic State, alliance leaders will also focus during their two days in Warsaw on the future of the NATO-led campaign in Afghanistan.
Obama's first NATO summit early in 2009 centered on the new president's desire to follow through on a key campaign pledge, to bring the war to an end, beginning with new commitments of support from alliance members that ultimately brought about the NATO-led coalition in place in the country today.
The president's announcement Wednesday that he would delay his planned drawdown of U.S. forces in Afghanistan, while expected, was nonetheless likely to be greeted warmly by coalition partners that had committed military resources to Afghanistan long beyond 2017.
Obama's 20th trip to Europe will extend beyond Poland. On Saturday, he will travel to Spain for a mix of business and pleasure, with a sightseeing trip through Seville followed by meetings Monday in the Spanish capital, Madrid.
Administration officials noted that Spain was the largest European nation Obama had yet to visit as he arrives at the midpoint of his eighth year in office.
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12:45 p.m.: This article was updated with details about a new NATO intelligence post.