WASHINGTON - (AP) - President Barack Obama is offering Congress and an anxious public his first detailed accounting of his rationale for U.S. military involvement in Libya and perhaps an answer to the burning question: What's next?

His speech, set for 7:30 p.m. tonight, comes after the administration scored an important diplomatic victory with the approval by NATO ambassadors on Sunday of a plan for the alliance to assume from the U.S. command of all aerial operations, including ground attacks.

That will help Obama assure Americans he can deliver on his vow that the United States will be a partner in the military action against Libya, but not from the driver's seat. Bickering among NATO members delayed the process.

Ahead of tonight's speech, Obama and his top national security officials worked to set the stage for the address - Obama in his weekly radio and Internet address and Secretary of State Hillary Rodham Clinton and Defense Secretary Robert Gates with appearances on Sunday television news shows.

But as they made the rounds, neither Clinton nor Gates could say for sure how long the U.S. mission would last or lay out an exit strategy.