LONDON — With U.S., British and French forces now fully engaged in attacking Moammar Gadhafi's military in Libya from the air and sea, and the chairman of the U.S. Joint Chiefs of Staff declaring that a no-fly zone is now in effect, the question becomes: How does this end?

The United Nations Security Council resolution that authorized the attacks defines the goal as a ceasefire that stops Gadhafi from assaulting his people.

President Barack Obama on Thursday added to that by saying that Gadhafi "must stop his troops from advancing on Benghazi, pull them back from Ajdabiya, Misrata, and Zawiya" — three cities that had at one-time been under rebel control — "and establish water, electricity and gas supplies to all areas."

"Humanitarian assistance must be allowed to reach the people of Libya," the president added.

But whether such a ceasefire could leave Gadhafi in power remains an open question. Neither the U.N. nor Obama have said explicitly that Gadhafi must be removed from power, though Obama had called for Gadhafi to step down previously, saying he'd lost all legitimacy to govern.

On Sunday, U.S. Under Secretary of Defense Michele Flournoy used the same formulation in an interview with the BBC, implying that nothing short of a Gadhafi departure from power was acceptable. "He's lost his legitimacy," she said.

Still, Flournoy was unwilling to state explicitly that Gadhafi had to go for the U.S.-led campaign to end. "It's too early to speculate as to where this ends up," she said.

Adm. Mike Mullen, the chairman of the U.S. Joint Chief of Staff, was similarly reluctant to make any long-term predictions in an interview with ABC News.

Describing the military objective as "limited," he dodged a question about whether the no-fly zone over Libya might remain in place for as long as the U.S. enforced a similar zone over Iraq — 12 years. "Circumstances will drive where this goes," he said.

That troubles some military analysts, who worry that the West's urgent action over the weekend isn't backed by planning for what sort of Libya will be left behind when the aerial campaign stops.

It also troubles leading lawmakers on Capitol Hill.

"Before any further military commitments are made, the administration must do a better job of communicating to the American people and to Congress about our mission in Libya and how it will be achieved." House of Representatives Speaker John Boehner, R-Ohio, said Sunday.

"I think we're seeing the opening shot of a fairly long campaign," said retired Royal Navy Rear Adm. Chris Parry, a former top planner for Britain's Defense Ministry. Calling Saturday's aerial strikes against Libya as a "something-must-be-done strategy," Parry said he'd seen no "evidence of a long-term strategy."

The U.N. resolution "only takes us so far," he said. "Some thought has got to be given to what comes next."

Paul Pillar, a professor at Georgetown University in Washington who spent nearly three decades as a senior U.S. intelligence analyst, said on its face, the U.N. resolution offers no formula for ending the West's military obligations. "If the mission is to protect Libyans, this is a mission that inherently has no end," he said, as long as Gadhafi remains in power.