NATO was formed in 1949 to protect Europe against the growing power of the Soviet Union. When the organization's first secretary general, Hastings Ismay, took the helm of the alliance in 1952, he proclaimed its goal to be "to keep the Russians out, the Americans in, and the Germans down."
But the common enemy that NATO faced during the Cold War has long been replaced by changing alliances, the specter of terrorism and security threats that can be carried out through traditional military force or through broadband cables and gas pipelines. NATO faces a wide and vastly different array of threats today, both to individual member states' national security interests and to peace on the continent.
When cyber-attacks shut down Estonia's Internet infrastructure in May 2007, officials turned to NATO to fix the damage and put tighter security measures in place. The attack pushed NATO to approve an official policy on cyber defense in 2008, which created a set of best practices to help members protect themselves, as well as a Cyber Defense Management Authority to support individual allies in defending against cyber-attacks upon request.
Energy resources also have emerged as a new sticking point. Last January, Russia flexed its political power by cutting off its natural gas supply to Europe amid a dispute with potential NATO member Ukraine over gas prices and the payment of other debts. The pipeline disruption left millions in central Europe without heat in the bitter cold of winter.
While Russia is not a formal member of NATO, the Allies and Moscow have a framework for consultation and cooperation via the NATO-Russia Council (NRC) which was established in 2002.
Russia has bristled as former Soviet states and countries along its borders have sought membership in NATO, raising new questions about the organization's future and the NATO-Russia relationship.
The August 2008 Russian-Georgian war also dredged up old insecurities. The dispute between Georgia, another possible candidate for NATO membership, and Russia hinged on control of the breakaway Georgian provinces of Abkhazia and South Ossetia.
Cyber-attacks also emerged during the Russia-Georgia war. Georgian government Web sites -- including the president's office, the parliament, and the foreign ministry -- were defaced with anti-Georgian or pro-Russian images.
According to NATO's Web site, the alliance suspended formal meetings of the NRC and cooperation in some areas in the aftermath of the Russia-Georgia conflict. In subsequent meetings, the latest in March 2009, NATO ministers agreed to re-engage Russia in formal meetings after the April NATO summit.
The debate over whether to enlarge NATO's membership -- and the shape of Russia's role-- continues to be a core issue.
Ben Tonra, professor of international affairs at the Dublin European Institute says NATO enlargement is very problematic because the prospect of eastward expansion is so inflammatory to Russia. With Georgia, Belarus and Ukraine seeking alliance membership, Russia perceives an "encroachment on its sphere of influence," Tonra said.
Albania and Croatia officially join the alliance this week, bringing the number of member states to 28, a jump from the 12 nations that signed the original treaty in Washington, D.C., on April 4, 1949.
While further NATO enlargement is a topic on the summit discussion agenda, many analysts believe that it will be put on hold in favor of focusing on areas in which Russia and the U.S. have begun to share common ground, like nuclear and traditional arms control and Iran.
Charles Kupchan, a senior fellow for Europe studies at the Council on Foreign Relations and professor of international affairs at Georgetown University, said that while NATO will officially keep the door open to Ukraine, Georgia and other potential member states, the enlargement issue is currently too "contagious and poisonous" for the Russians.
"I think NATO is slowing the pace of enlargement down, and taking it from the front to the back burner as a way of prompting a new effort to reach out to Russia," Kupchan said. It's not plausible to talk about "hitting the reset button" on U.S.-Russia relations -- Secretary of State Hillary Clinton's phrase -- while backing Ukraine's and Georgia's membership bids, he said.
"The other aspect of the enlargement debate is where Russia belongs," Kupchan added. "Should it be treated as a potential member of the Euro-Atlantic community, or do Europe's imagined borders really end before Russia begins?"
The selection of a new NATO secretary general to succeed Jaap de Hoop Scheffer is also poised to prove contentious due to Turkey's resistance to the front-runner for the post, Danish Prime Minister Anders Fogh Rasmussen. Rasmussen refused to apologize after a Danish newspaper cartoonist angered Muslims and prompted worldwide riots with a 2006 series of offensive images of the Prophet Mohammed. And as NATO looks to make a fresh start, some view Rasmussen as tied too closely to the past.
Rasmussen has enjoyed the support of the main European powers and the United States in the run-up to the NATO meeting.
Other names mentioned for consideration include Canadian Defense Minister Peter MacKay and former British Defense Secretary Des Browne. An earlier hopeful, Polish Foreign Minister Radoslaw Sikorski, ruled himself out of the running on Friday, the Associated Press reported.
"There are a lot of great candidates available, although a Canadian would make a lot of Europeans nervous," said Stan Sloan, visiting scholar at the Rohatyn Center for International Affairs at Middlebury College. "It's hard to predict who it will be, but if the Turks really want to block Rasmussen, they can. They're willing to put their veto behind it."
Of all the items on the summit's packed two-day agenda, Afghanistan is by far the most divisive. "One of the biggest challenges that NATO faces right now is coming to a consensus on the right balance between performing missions like Afghanistan -- expeditionary missions far from Europe -- and its need to re-stabilize the European continent and its periphery," said Ronald Asmus, executive director of the Transatlantic Center of the German Marshall Fund.
The Russia-Georgia conflict was also a "wake-up call to the continent" that Europeans were naive to believe the continent was stable, Asmus said. "It made people stop to think, 'Maybe we can't send all our best troops to Afghanistan unless everything at home is OK,'" he said.
One of President Obama's most important tasks is to provide a solid outline of the Afghanistan-Pakistan plan he unveiled last week in Washington. "We need to have a clear perspective on goals in Afghanistan and the cost of everything -- especially the cost," said Francois Gere, head of the French Institute of Strategic Analysis. "The West is facing a major economic crisis, and people who have deep concerns about their daily life and employment can not understand why a government would spend euros or dollars on a mission which is not clear," Gere said.
There is also a deepening divide among those member states who have sent more troops to Afghanistan under the NATO-led coalition force, such as the UK, Germany, Italy and Canada, and those who have sent relatively few. "Some allies are being exposed to much greater risk than others, and some allies are dying and some are not," Asmus said, adding that the alliance is "starting to fray on the issue of risk-sharing."President Obama's greatest strength lies in the openness much of Europe is demonstrating toward him, Asmus said. "There's a deep desire to re-embrace America, but there is still an underlying resentment and anger that is mixed in. This is really the first installment in his repair job of fixing the transatlantic alliance."