What “rules of war” should apply to Predator drones, the eerily efficient weapons that cruise the skies and target adversaries with the precision of a sharpshooter’s bullet? It’s an urgent question — not simply for the United States, which is expanding its use of drones, but for dozens of other nations that may soon use them to target their own “bad guys.”

Although drones have been controversial abroad, there has been relatively little public debate about them in America. That’s partly because U.S. officials usually won’t discuss their operations, which are highly classified. But officials affirm privately that they have been highly effective against al-Qaeda’s leadership in the tribal areas of Pakistan — and that they are being used in Yemen and Somalia in an escalating campaign against al-Qaeda affiliates there.

These weapons, which project power without risking “boots on the ground,” can become addictive. According to a report last year by a U.N. special rapporteur, more than 40 countries now have drone technology, and nations seeking to arm drones with missiles include Israel, Russia, Turkey, China, India, Iran, Britain and France.

“We have to be extremely careful and prudent about how we use this technology. It’s very efficacious in killing terrorists, but there are significant risks of blowback from its widespread use that could harm our counterterrorism efforts,” argues Jack Goldsmith, a Harvard Law School professor who served in the George W. Bush administration’s Justice Department.

A drone debate, of sorts, took place behind the scenes in preparation for a speech last Friday by John Brennan, the White House counterterrorism adviser. He argued that U.S. legal authority to use force against al-Qaeda wasn’t “restricted solely to ‘hot’ battlefields like Afghanistan” but could be expanded to other theaters “without doing a separate self-defense analysis each time.”

On the eve of Brennan’s speech, the New York Times reported a split within the Obama administration. The Pentagon’s general counsel, Jeh Johnson, was said to have urged broad targeting against groups anywhere that are aligned with al-Qaeda, while the State Department’s legal adviser, Harold Koh, reportedly recommended a more limited rule that, outside Afghanistan and Pakistan, would authorize targeting only individuals actually plotting to strike America.

These disagreements were resolved by Brennan’s speech, which took a hard-line view. Brennan conceded that some key allies, though “converging” toward U.S. legal arguments, “take a different view of the geographic scope of the conflict, limiting it only to the ‘hot’ battlefields.”

Here’s the real question, according to current and former officials: As the United States steps up Predator attacks over Yemen and Somalia, should it adopt the same “signature” targeting it uses over Pakistan? Under this approach, the drones can strike al-Qaeda training camps and fighters not on the list of specific targets compiled by the CIA. The signature approach is more aggressive, but it risks creating what terrorism analyst David Kilcullen calls “accidental guerrillas” — and thereby widening the war.

To understand the debate, some background is useful. The CIA’s legal authority (it conducts attacks over Pakistan and will probably have similar responsibility in Yemen and Somalia) dates to a lethal covert-action “finding” signed days after Sept. 11, 2001. The CIA’s Counterterrorism Center compiles a list of approved targets, usually numbering less than several dozen, based on intelligence that they pose a serious, continuing threat to the United States. That list is reviewed every six months, and names come on and off.

Legal review is done by the CIA’s general counsel, who in turn consults with the White House counsel. Signature targeting was added in 2008, using the same 2001 presidential finding, which was renewed by President Obama in 2009. The rules call for notifying the National Security Council (including the attorney general) if a U.S. person is a target. Such a broader review apparently took place when Anwar al-Aulaqi, a U.S. citizen in Yemen who is a seniorofficial in al-Qaeda in the Arabian Peninsula, was added.

Here’s the real problem with drones. They may indeed reduce collateral damage, as their proponents argue, because of their precision and surveillance. And America’s growing use of them against al-Qaeda may be legal under international law that allows self-defense. But what is legal isn’t always wise.

A world where drones are constantly buzzing overhead — waiting to zap those deemed threats under a cloaked and controversial process — risks being, even more, a world of lawlessness and chaos. Drones have been America’s best weapon against al-Qaeda, but one to use sparingly — against people U.S. intelligence knows are seeking to kill others.