The United Kingdom's vote to exit the European Union, which Obama campaigned ardently against, has thrown the bloc into chaos. The undercurrents that pushed "Brexit" through, like opposition to immigration and trade deals, could wind up blemishing Obama's own legacy.
And Russian aggression also is a growing concern for the alliance, with Defense Secretary Ash Carter addressing the issue and the need for increased NATO deterrence when he spoke to reporters on his flight to Poland Thursday.
With a still-murky path toward a final Brexit split, leaders like outgoing British Prime Minister David Cameron and German Chancellor Angela Merkel -- the two heads of state to whom Obama is closest -- will arrive at this week's summit eager to hear from the President directly. Aides say Obama will also look to gain a better sense of how and when the British exit will proceed.
In addition, a spate of global terror attacks in the final week of Ramadan, including in the capital of NATO member Turkey, will provide the alliance with new challenges.
Doug Lute, the U.S. permanent representative to NATO, described the summit as coming at an "inflection point" for the alliance that hasn't been seen since in two decades.
He cited Russian aggression, the conflicts in Syria and Iraq, and the rise of ISIS as fueling the sense the summit is occurring at a critical moment.
"All these factors in multiple directions combine to really mark this as different in NATO's long history," he said. "By my count, 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 '91. So this is a bit of a historic point, and it's a great point for the summit, especially as the President wraps up his tenure as the leader of the alliance."
Obama was set to meet with top European leaders, including the presidents of the European Council and the European Commission, upon arriving in Poland.
For Obama, who has worked during his two terms to "pivot" foreign policy away from Europe and toward the Asia Pacific, the meeting represents how slow progress can be in shifting from the traditional areas of focus for the United States. NATO's standoff with Russian President Vladimir Putin over aggression in Ukraine has revived Cold War-era suspicions, disappointing a White House intent on fostering global cooperation.
Obama spoke with Putin on Wednesday ahead of his departure for the summit. The Russian leader has grown increasingly hostile toward NATO as the alliance has bolstered its troop levels in Eastern Europe, saying in May he would respond "appropriately."
Furthermore, Obama's announcement Wednesday that he would leave more U.S. troops in Afghanistan when he departs office than originally planned underscored the difficulty he's faced in moving past the global conflicts of his predecessors.
An administration official said Obama made the Afghanistan announcement ahead of the NATO summit to provide "clarity about our intentions" heading into the meeting. NATO, with U.S. backing, has led the mission to train Afghan security forces since the end of combat there in 2014.
In Warsaw, Obama will press fellow leaders to increase financial contributions for the Afghanistan mission, and to maintain pressure on Putin as Russian provocations persist in Eastern Europe.
During past visits to Europe, including during 2014's NATO summit in Wales, Obama's goal had been to reassure allies the United States was committed to upholding its treaty obligations in protecting them against border incursions.
Gen. Philip Breedlove, who retired earlier this year as the head of U.S. European Command and as NATO Supreme Allied Commander in Europe, said he expected the focus of the summit to shift from reassurance toward actually deterring Russian aggression through the deployment of NATO forces in Eastern Europe.
"Deterrence ... a step on the journey, and deterrence will be enhanced as that journey goes along," Breedlove said during a talk at the Center for Strategic and International Studies in late June. "It could be one of the biggest deliverables in Warsaw."
Carter told reporters Thursday that NATO is moving toward greater deterrence, something it hasn't had to do in a quarter century. He added that NATO still works with Russia where it can find common ground.
In June, NATO announced it was deploying four multinational battalions to Estonia, Latvia, Lithuania and Poland amid scaled-up tensions with Moscow.
Obama also hopes to pressure European leaders to maintain sanctions on Russian as punishment for the Ukraine incursion, though economic uncertainty in Europe could make that argument harder, given closer business ties to Russia in Europe than in the U.S.
U.S. officials have downplayed the security risks that Britain's decision to exit the EU could cause. But because the precise terms of Britain's exit from the union are still being determined, it's not yet clear how the vote will affect security ties within the continent or with the United States.
Obama insisted in Canada at the end of last month that Britain and Europe proceed carefully -- even as leaders like Merkel are pushing for a faster resolution in the interest of providing greater certainty.
"Everybody should catch their breath, come up with a plan and a process that is orderly, that's transparent, that people understand, and then proceed understanding that both sides have a stake in getting this right," Obama said.
In Warsaw, Obama isn't likely to alleviate much of the post-vote anxiety that roiled markets and has prompted calls for independence votes from Northern Ireland to Catalonia. But he will push leaders to develop closer cooperation between NATO and the EU, which have historically operated separately, despite overlapping memberships.
"If anything, it just strengthens the need for enhanced NATO-EU cooperation," said NATO Secretary General Jens Stoltenberg of the Brexit vote.
"Both the European Union and NATO face a new security environment with new threats: hybrid, terrorism, instability," he added. "Neither the European Union nor NATO possesses all the tools to respond. So, therefore, we have to work together."