Waging war, says Yoo, is unlike administering criminal justice in one decisive particular. The criminal justice system is retrospective: It acts after a crime. A nation attacked, as America was on Sept. 11, goes to war to prevent future injuries, which inevitably involves probabilities and guesses.

Today’s war is additionally complicated by the fact that, as Yoo says, America’s enemy “resembles a network, not a nation.” Its commanders and fighters do not wear uniforms; they hide among civilian populations and are not parts of a transparent command-and-control apparatus. Drones enable the U.S. military — which, regarding drones, includes the CIA; an important distinction has been blurred — to wield a technology especially potent against al-Qaeda’s organization and tactics. All its leaders are, effectively, military, not civilian. Killing them serves the military purposes of demoralizing the enemy, preventing planning, sowing confusion and draining the reservoir of experience.

Most U.S. wars have been fought with military mass sustained by economic might. But as Yoo says, today’s war is against a diffuse enemy that has no territory to invade and no massed forces to crush. So the war cannot be won by producing more tanks, army divisions or naval forces. The United States can win only by destroying al-Qaeda’s “ability to function — by selectively killing or capturing its key members.”