WASHINGTON – U.S. diplomatic outposts reopened throughout the Middle East on Sunday, easing the sense of imminent danger that has preoccupied the Obama administration since it learned of a possible terrorist attack from communications between two high-ranking officials of Al-Qaida nearly two weeks ago.

But the one embassy that remained closed — in Sanaa, the capital of Yemen — underscored the challenges President Obama faces in trying to wind down the nation's decadelong campaign against Al-Qaida and its associates and reshape the nation's counterterrorism strategy.

In response to the latest threat, the United States has unleashed a barrage of drone strikes in that impoverished country, but it is unclear to what extent it has reduced the persistent and deadly threat from an increasingly decentralized Al-Qaida organization. The United States has carried out nine strikes in Yemen since July 28, broadening its target list beyond the high-level leaders it has always said are the main objective of the attacks.

Senior U.S. counterterrorism and intelligence officials say the lack of certainty about the effectiveness of the latest drone strikes is a sobering reminder of the limitations of U.S. power to deal with the array of new security threats that the turmoil of the Arab Spring has unleashed.

These doubts come even as lawmakers in Washington debate whether to restrict the surveillance activities of the National Security Agency.

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