AS THE FIGHTING in Libya worsened on Wednesday, the Obama administration was sending conflicting signals. Senior officials were rightly warning of the huge stakes for U.S. security and for the future of the Middle East — while at the same time seeming to back away from action that might contribute to the U.S. aim of ending the dictatorship of Moammar Gaddafi.

Secretary of State Hillary Rodham Clinton told a Senate committee that “one of our biggest concerns is Libya descending into chaos and becoming a giant Somalia,” where al-Qaeda could find refuge. Earlier this week she said Mr. Gaddafi “has to go.” She was right on both counts. Prolonged fighting in Libya, which began to seem more likely this week as the Gaddafi regime mounted counterattacks against opposition-held cities, could quickly lead to anarchy in a country that has already been a source of fighters for al-Qaeda.

Should Mr. Gaddafi manage to survive and regain control, the consequences would be equally terrible. In addition to unthinkable bloodshed and suffering for Libyans, the dictator would demonstrate that U.S. and European demands for his downfall were meaningless — and offer an example to other Arab autocrats facing popular uprisings.

Given those circumstances, it only makes sense that Ms. Clinton and other officials began talking this week of tangible measures to curtail Mr. Gaddafi’s military operations, such as a no-fly zone — a step requested by Libya’s own delegation to the United Nations. On Wednesday the regime again dispatched planes to attack opposition positions, and it may also be using aircraft to transport troops or foreign mercenaries.

Part of the Libyan air force has already defected, and the opposition has captured several of its bases — so grounding the remaining warplanes would appear to be a manageable undertaking for U.S. or NATO forces. Yet in his own congressional testimony Wednesday, Defense Secretary Robert M. Gates did his best to dump cold water on the idea — claiming that it could not be done with one aircraft carrier and would require “an attack on Libya to destroy the air defenses.”

While the Pentagon’s view cannot be taken lightly, this assessment sounded exaggerated. U.S. and allied warplanes maintained a no-fly zone over Iraqi Kurdistan for more than a decade without eliminating Saddam Hussein’s air defenses. Similarly, Western planes could defend the areas of Libya under opposition control from air attack without taking on every air defense battery in the relatively small territories under Mr. Gaddafi’s sway. The mere threat of encountering U.S. warplanes could serve to ground what remains of Libya’s air force.

No U.S. military action should be undertaken in Libya without a careful assessment of the risks. A clear appeal from an opposition authority, and support from Arab or European states, are also needed. But the United States should not settle on inaction because of inflated assessments of the regime’s remaining capabilities, or resistance from U.N. Security Council members such as Russia. If indeed the stakes in Libya are as Ms. Clinton and others describe them, the United States must do what it can to help bring about Mr. Gaddafi’s downfall.