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President Barack Obama and the leaders of NATO's 27 other member nations open a two-day summit Friday aimed at finding ways to keep the Cold War alliance relevant in the 21st century with revamped roles including ballistic missile defense, anti-piracy patrols and counterterrorism.

But the meeting is being overshadowed by the escalating war in Afghanistan, where the alliance is struggling to contain Taliban militants.

NATO officials say they expect unanimous support from the allies for Obama's plans for a new, expanded missile defense system in Europe that would be based on an existing shield meant to defend military units from attack. The U.S. already has a missile defense system based mainly in North America, and it is planning one for its European allies.

But Obama will face tough questions from U.S. allies on his exit strategy in Afghanistan. He will also meet with leaders of the European Union on Saturday to defend his preference for stimulus spending at a time when many European nations are enacting economic austerity measures.

On Saturday, the leaders are expected to endorse a plan by Gen. David Petraeus, the top U.S. and NATO commander in Afghanistan, to start handing over responsibility for security in some areas of Afghanistan to government forces in 2011. The plan is to end the alliance's combat role by 2014 if conditions on the ground allow, but to retain significant forces in the country after that to train and advise the Afghan army and police.

"It will be a very important moment," Adm. Giampaolo di Paola, NATO's top military official, told The AP on Friday. "The start of transition is also testimony that the alliance is succeeding in Afghanistan."

The alliance has 140,000 troops in Afghanistan, two-thirds of them Americans. The government's security forces are being built up to just over 300,000 members. Their Taliban opponents are estimated to number up to 30,000 men.

NATO's newly expanded anti-missile shield would cost euro200 million ($273 million) over the next 10 years, said NATO Secretary-General Anders Fogh Rasmussen, who also wants Russia to cooperate in the project. Despite claims by protesters that debt-plagued Europe can't afford it amid austerity cuts, alliance officials insisted the project is worth it.

"We think it's a good thing to have a missile defense system which is NATO-based," Britain's Defense Secretary Liam Fox told BBC Radio 4's Today program. "That provides us with communal protection over the years ahead, it's cost-effective for us, and there are some 30 countries now which either have or are developing ballistic missiles that this will give us protection from."

Founded in 1949 to counter the threat of a Soviet invasion, the 28-member alliance is in the midst of a mid-life crisis as it searches for relevance almost 20 years after the collapse of its communist rival.

Other elements of NATO's new mission statement expected to be adopted Friday include new roles such as cyber-warfare and missions outside NATO's traditional area in Europe, such as anti-piracy patrols off the Somali coastline.

NATO's previous strategic concept focused mainly on its peacekeeping role in places like Bosnia and Kosovo. It was adopted in 1999, soon after the end of the Cold War and before the Sept. 11, 2001, attacks in the United States forced the alliance to take on missions such as counterinsurgency warfare in Afghanistan.

The new document will warn European governments not to slash defense spending at a time of economic crisis, because of the growing discrepancy in military capabilities between the United States and Europe's NATO members. Most European nations are not even meeting the minimal requirement of devoting 2 percent of their GDP to defense.

America's latest defense budget of over $710 billion dwarfs the combined annual military expenditures of its European allies, which total about $280 billion. Despite the added expenses of the Afghan war, many European capitals already have announced further military cuts.

Failure in Afghanistan could leave alliance members questioning whether NATO's nation-building goals in the embattled country have been worth the cost, and whether they will support similar missions in the future, a RAND Corporation study said Friday.

Allied commanders have highlighted successes this year against Taliban insurgents in Helmand and Kandahar provinces, to emphasize that transition is ready.

But allied casualties have also reached record levels of some 650 dead this year, and the Taliban have spread out into parts of Afghanistan where they were not active before.