European officials, when promised anonymity, wring their hands that this is the first NATO operation since the creation of the alliance a half-century ago in which the United States has declined to take the lead. Some former Clinton administration officials have uttered similar concerns, along with Republican critics like Senator John McCain, who made his point last week by flying into Benghazi, Libya, and visiting with the rebels, whom he immediately declared his “heroes.” (His enthusiasm is apparently not broadly shared in the Republican Party: At the same moment Mr. McCain was cheering on Colonel Qaddafi’s opponents, Representative Michele Bachmann of Minnesota, the Tea Party favorite weighing a run for the presidency, declared that the president was foolish to support rebels who include “elements of Al Qaeda in North Africa and Hezbollah.”)

Yet the question may be not whether the United States leads, but whether it puts its credibility on the line by seeming to enter the conflict half-heartedly. “The problem is the gap between U.S. objectives and what we are willing to do to accomplish them,” Richard N. Haass, the president of the Council on Foreign Relations, who left the Bush administration in part because of his objections to the drive to invade Iraq, said on Friday. “We either have to do a lot more — and the Predators were a step in that direction — or go for a cease-fire and live with the fact that Qaddafi could be in place for some time.”

In short, Mr. Haass is making the case that President Obama is violating the Powell Doctrine: If you elect to use the United States military, you must do so with such overwhelming force that there is no doubt of the outcome.