President Franklin Roosevelt was truly astonished when told by a reporter that Adm. Isoroku Yamamoto, architect of the Pearl Harbor attack, had been shot down by U.S. planes over a Pacific island after Americans decrypted Yamamoto’s flight plans. FDR had encouraged this “targeted killing” — destroying a particular person of military importance — a phrase that has become familiar since Israel began doing this in 2000 in combating the second Palestinian intifada.

But was the downing of Yamamoto’s plane an “assassination”? If British commandos had succeeded in the plan to kill German Gen. Erwin Rommel in Libya in 1941, would that have been an assassination? If President Reagan’s 1986 attack on military and intelligence targets in Libya, including one that Moammar Gaddafi sometimes used as a residence, had killed him, would that have been an assassination? What about the November 2001 CIA drone attack on a Kabul meeting of high-level al Qaeda leaders that missed Osama bin Laden but killed his military chief? An old executive order and a new technology give these questions urgent pertinence.

Executive Order 12333, issued by Reagan in 1981, extended one promulgated by Gerald Ford in 1976 — in response to revelations about CIA attempts to assassinate Fidel Castro — and affirmed by Jimmy Carter. Order 12333 says: “No person employed by or acting on behalf of the United States Government shall engage in, or conspire to engage in, assassination.” What, then, of the Navy SEALs who killed bin Laden? The new technology is the armed drone, which can loiter over the suspected location of an important enemy person and, in conjunction with satellite imagery, deliver precision-guided munitions in a matter of minutes.

Fortunately, John Yoo of the Berkeley School of Law has written a lucid guide to the legal and moral calculus of combating terrorism by targeting significant enemy individuals. In “Assassination or Targeted Killings After 9/11” ( New York Law School Law Review, 2011/12) Yoo correctly notes that “precise attacks against individuals” have many precedents and “further the goals of the laws of war by eliminating the enemy and reducing harm to innocent civilians.” And he clarifies the compelling logic of using drones for targeted killings — attacking a specific person rather than a military unit or asset — in today’s “undefined war with a limitless battlefield.”

To be proper, any use of military force should be necessary, as discriminating as is practical, and proportional to the threat.

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 9/11, goes to war to prevent future injuries, which inevitably involves probabilities and guesses.

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.”

After the terrorist bombings of two U.S. embassies in East Africa in 1998, the Bill Clinton administration launched cruise missiles against suspected terrorist camps in Afghanistan, hoping bin Laden was there. If the missiles had killed him, would this have been improper? In March 2003, in the hours before the invasion of Iraq, the George W. Bush administration, thinking it knew where Saddam Hussein was, launched a cruise missile strike against one of his compounds. Was it wrong to try to economize violence by decapitating his regime? Would it have been morally preferable to attempt this by targeting, with heavy bombing, not a person but his neighborhood? Surely not.