Today, there is little political appetite for the risk, cost and especially the long timelines required by counterinsurgency doctrine, which involves building societies and governments to gradually take over the battle against insurgents and terrorists within their borders.

The apparent simplicity of a drone aloft, with its pilot operating from the United States, can be misleading. Behind each aircraft is a team of 150 or more personnel, repairing and maintaining the plane and the heap of ground technology that keeps it in the air, poring over the hours of videos and radio signals it collects, and gathering the voluminous intelligence necessary to prompt a single strike.

Air Force officials calculate that it costs $5 billion to operate the service’s global airborne surveillance network, and that sum is growing. The Pentagon has asked for another $5 billion next year alone for remotely piloted drone systems.

Yet even those costs are tiny compared with the price of the big wars. A Brown University study, published in June, estimates that the United States will have spent $3.7 trillion in Afghanistan and Iraq by the time the wars are over.

The drones may alienate fewer people. They have angered many Pakistanis, who resent the violation of their country’s sovereignty and the inevitable civilian casualties when missiles go awry or are directed by imperfect intelligence. But while experts argue over the extent of the deaths of innocents when missiles fall on suspected terrorist compounds, there is broad agreement that the drones cause far fewer unintended deaths and produce far fewer refugees than either ground combat or traditional airstrikes.

Still, there are questions of legality. The Obama administration legal team wrestled with whether it would be lawful to make Mr. Awlaki a target for death — a proposition that raised complex issues involving Mr. Awlaki’s constitutional rights as an American citizen, domestic statutes and international law.

The Justice Department’s Office of Legal Counsel eventually issued a lengthy, classified memorandum that apparently concluded it would be legal to strike at someone like Mr. Awlaki in circumstances in which he was believed to be plotting attacks against the United States, and if there was no way to arrest him. The existence of that memorandum was first reported Saturday by The Washington Post.