The exit plan leaves Afghanistan facing grave uncertainties: Can its forces take over the main fight against the Taliban-led insurgency and avert a resurgence of the mayhem that raged before the 2001 U.S.-led invasion, allowing al-Qaeda to establish a virtual parallel state in the country?
The strategy also means that Afghan President Hamid Karzai's weak government will conduct a 2014 election for his successor as the last U.S. and allied combat forces are leaving, potentially jeopardizing the country's first peaceful transfer of political power.
Obama and the 27 other leaders of the alliance, however, are confronting intense pressure from war-weary citizens to bring their troops home and focus on restoring growth and creating jobs amid Europe's intensifying financial crisis and overstretched national budgets.
For Obama, the strategy has required a delicate balancing act in the midst of a re-election fight clouded by a weak U.S. economic recovery: He must bow to the majority of Americans, who oppose the war, while reassuring Karzai and his people that the United States won't abandon them as it did after the 1979-89 Soviet occupation.
"I think it is the appropriate strategy whereby we can achieve a stable Afghanistan that won't be perfect," Obama said. "We can pull back our troops in a responsible way, and we can start rebuilding America and making some of the massive investments we've been making in Afghanistan here back home, putting people back to work, retraining workers, rebuilding our schools."
Responding to concerns that the withdrawal of U.S. combat power may be premature, Obama acknowledged that the Taliban remains "a robust enemy." But he added that he was satisfied with assessments by White House military advisers who served in Afghanistan that the strategy will work.
"I can't afford a whitewash. I can't afford not getting the very best information in order to make good decisions," he said. "The Afghan security forces themselves will not ever be prepared if they don't start taking ... responsibility" for the country's security.
The U.S. intelligence community, however, has warned that the gains made by the 2010 surge of about 30,000 American troops into the Taliban strongholds of southern Kandahar and Helmand provinces could be lost as those forces are withdrawn at the end of this summer.
The summit also ended without a resolution to a dispute between the United States and Pakistan over reopening NATO supply routes through Pakistan to Afghanistan.
The strategy adopted on the final day of NATO's largest-ever gathering calls for the alliance's 130,000-strong U.S.-led International Security Assistance Force to complete a handover to Afghan security forces of the lead role in combat operations across the country of 31 million by summer 2013.
The coalition will assume a supporting role as it completes a phased withdrawal by the end of 2014. The White House and Pentagon are still deliberating over the number of U.S. troops who would remain after 2014 to train Afghan troops and police, provide logistical and intelligence help, and conduct raids against al-Qaeda.
The United States succeeded in winning from allies pledges to provide nearly all the $1.3 billion in annual funding it was seeking to support 352,000 Afghan forces through 2017, when that number of troops is to drop to 228,500, U.S. officials said.
Washington is to provide $2.3 billion per year and Afghanistan $500,000.
But major hurdles remain. They include whether Kabul can fulfill its pledges in the new strategic agreement to guarantee free and fair elections - Karzai's 2009 election to a second term was fraught with fraud and record violence - along with cracking down on corruption and ensuring the rights of women and minorities.
Even more uncertain is whether Afghan security forces will be able to take over the leading role in the fight against the Taliban-led insurgency, and whether a U.S. effort to broker peace talks between Kabul and the insurgents can be revived since the Taliban withdrew in March.
"At the end of the day, our strategy going forward is based on a series of very aggressive wishful assumptions," said Michael Waltz, a senior fellow at the New America Foundation, who served as a U.S. special forces commander in Afghanistan. "This is all assuming a hell of a lot of risk."