Any additional airstrikes would require the president’s approval. Even then, officials indicated that such air support would be hard to sustain over a lengthy period because of the enormous logistical effort that would be necessary given the American withdrawal. The United States will leave all its air bases in Afghanistan by next month, and any airstrikes would most likely have to be launched from bases in the Persian Gulf. A potential fall of Kabul is the crisis most likely to lead to military intervention after U.S. troops leave, officials said. Intervening to protect Kandahar, Afghanistan’s second-largest city, would be far less certain, one official said. Encroaching Taliban forces have increasingly threatened several other urban hubs in almost every corner of the country in recent months. The discussion suggests the degree of concern in Washington about the ability of Afghanistan’s military to hold off the Taliban and maintain control of Kabul and other population centers.