Updated | March 25 When the United States and its allies began bombing Libya five days ago, there were plenty of easy marks to be had.

“Early on, we had many fixed sites which we knew we could target,” General Carter Ham, commander of U.S. Africa Command, said in a briefing at a naval air base in Italy Thursday. “There are not so many of those left.”

The allied bombing campaign, with its hundreds of Tomahawk missiles and fighter jets, has effectively wiped out the Libyans’ air defense systems and grounded the regime’s warplanes. What are left, experts say, are so-called “dynamic” targets, such as tanks and armed infantry that are moving from one city to the next.

And those are much harder to hit.

“It’s the most difficult target that we have because they are in and around the built-up areas of Libya,” Ham said. “Our concern for not causing civilian casualties makes that a particularly difficult target set for us.”

The challenge points up a broader concern for coalition forces as they transfer command of the allied mission in Libya from the U.S. to NATO: how to balance the narrow military objectives authorized by the United Nations Security Council with the more ambitious goal of ousting Moammar Gadhafi.

As military commanders have said repeatedly in recent days, their mission is not to target the Libyan autocrat. The U.N. Security Council resolution authorizes the use of force only to protect civilians. And Arab leaders, including NATO member Turkey, have expressed opposition to expanding the bombing campaign beyond its initial scope.

Now that NATO has agreed to take full command not only of the no-fly zone but of the overall mission in Libya, a more aggressive military campaign seems unlikely. As a senior administration put it Thursday, France, Britain and the U.S. — which have all called for the Gadhafi regime to go — are no longer calling the shots. “When it comes to deciding on what will or will not happen within a NATO operation, that gets done in Brussels,” the administration official said.

Those constraints present a political dilemma for President Obama and his European allies. If recent developments on the ground are any indication, air power alone may not be enough to fundamentally shift the balance of power in Libya. By their own admission, rebel forces remain disorganized and poorly trained, and Gadhafi’s loyalists have been relentless in their attacks on opposition strongholds such as Misrata and Ajdabiya.

Without a more aggressive bombing campaign, some experts fear, the fighting may well result in a stalemate.

“They have yet to alter the situation on the ground in any meaningful way,” said Nathan Hughes, director of military analysis at Stratfor, a global intelligence company. “There’s a no-fly zone, but everything else — except the assault on Benghazi — is continuing as before.”

Hitting those more difficult armed targets amassed on the edges of rebel-held cities, Hughes said, would require a more aggressive — and more dangerous — strategy. Allied fighter jets would have to fly closer to the ground, making themselves more vulnerable to surface-to-air attacks. Limited ground troops, such as special forces, might be necessary to train and assist the rebels. And missile strikes near population centers would be unavoidable, increasing the risk of civilian casualties.

As Hughes put it, military commanders essentially have two options: stepping up their attacks on Gadhafi’s ground forces to give the rebels an edge or pulling back to focus on enforcing the no-fly zone and avoid killing civilians.

“If they come down on one end of the spectrum, the coalition could itself end up inflicting civilian casualties. On the other end of the spectrum, they’re stuck watching civilian casualties being inflicted, and not doing anything,” Hughes said. “Neither of those are particularly great positions to be in.”

And if Gadhafi’s forces begin to withdraw from rebel-held cities, while fortifying their positions in parts of the country still under the regime’s control, the conflict could very well reach an impasse. “There’s very real room here for long-term stalemate, where Gadhafi remains in control of parts of the country,” Hughes said.

In fact, military leaders have been careful not to rule out that possibility.

“I could see accomplishing the military mission which has been assigned to me and the current leader would remain the current leader,” Ham said in a Pentagon briefing this week. “Is that ideal? I don’t think anyone would say that that is ideal, but I could envision that.”

Rebel leaders, meanwhile, are clamoring for an even more aggressive bombing campaign to help dislodge Gadhafi’s ground forces. They have formed a Provisional Transitional National Council and issued a statement Wednesday calling on coalition forces to “take all possible measures” to stop the regime’s attacks on opposition strongholds.

“The light arms of the pro-democracy fighters are no match to the Gadhafi’s brutal war machine which is being unleashed against them,” the rebel leaders said. “The Gadhafi regime has lost all legitimacy in the eyes of Libyans as well as the world community. It cannot be permitted to continue slaughtering its people because they ask for change. We implore you to act now.”

In the coming days, the challenge for NATO leaders will be to reconcile the military’s narrow mission — protecting civilians — with the broader political goal of ousting Gadhafi’s regime and allowing the pro-democracy movement to take root. The latter may well require an escalation of hostilities, something many NATO members, such as Turkey and Germany, oppose

“It’s not necessarily that easy of a thing to bring political and military goals in line, especially when you have a very limited stomach, or will power, for any sort of escalation,” Hughes said. “This could still backfire.”

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