Some critics, including a U.N. human rights watchdog group and Amnesty International, have urged the Bush administration to be more open about how it decides whom to kill and under what circumstances.

A U.N. report in the wake of the 2002 strike in Yemen called it "an alarming precedent [and] a clear case of extrajudicial killing" in violation of international laws and treaties. The Bush administration, which did not return calls seeking comment for this story, has said it does not recognize the mandate of the U.N. special body in connection with its military actions against Al Qaeda, according to Amnesty International.

"Zawahiri is an easy case. No one is going to question us going after him," said Juliette N. Kayyem, a former U.S. government counter-terrorism consultant and Justice Department lawyer. "But where can you do it and who can you do it against? Who authorizes it? All of these are totally unregulated areas of presidential authority."

"Paris, it's easy to say we won't do it there," said Kayyem, now a Harvard University law professor specializing in terrorism-related legal issues. "But what about Lebanon?"

Paul Pillar, a former CIA deputy counter-terrorism chief, said the authority claimed by the Bush administration was murky.

"I don't think anyone is dealing with solid footing here. There is legal as well as operational doctrine that is being developed as we go along," Pillar said. "We are pretty much in uncharted territory here."

This is not 'uncharted territory' at all. In 1986, the US bombed targets in Lebanon and killed 100 people - the justification was 'preemptive': "self-defense against future attacks". Article 51 of the UN Charter was invoked. (Ironically, when the Libyans captured two pilots who had bombed Libya and killed 37 people, it was used as an excuse to reject a Libyan offer to release those falsely accused of the Lockerbie bombing for trial in some neutral venue: to a judge nominated by the UN, at the Hague "under Scottish law" - exactly what transpired in the end, in however farcical circumstances). Aside from which, it is easy to see how this can be used as a tactic in other wars. Consider: the US bombs and kills "Islamist militants" in, say, Uzbekistan. Despite the fact that there is no threat to the US there, it can be justified as a strike against 'terrorism'. The opposition in Uzbekistan is largely not composed of Islamists, yet any such strike could be portrayed as an attack on 'Al Qaeda'. Subsequently, and surreptitiously, the tactic is used in other counterinsurgency campaigns, such as the one to crush the Maoists in Nepal, or Farc in Colombia (already, Dyncorp uses these planes to dump poison on coca growers in Colombia).