For weeks, rival Libyan militias had been pounding one another’s positions with artillery, mortar rounds and rockets in a desperate fight to control the international airport in the capital, Tripoli. Then suddenly, early Saturday morning, the fighting just stopped.

The pause came as United States military warplanes circled overhead, providing air cover for a predawn evacuation of the American Embassy’s staff. Apparently fearing the planes, the militias held their fire just long enough for the ambassador and her staff to reach the Tunisian border — a reminder to Libyans of how even their most powerful allies were incapable of putting out their incendiary feuds.

American officials said the evacuation was a temporary measure after fighting drew too close to the embassy. But, coming so soon after the withdrawal of other diplomatic missions, including the United Nations, the moment appeared to signal a defeat — for Libyans who had convinced themselves that the country would band together to save the revolution, and for the country’s Western allies, who sometimes acted as if Libya’s stability would take care of itself. […]