Washington ? Sen. Rand Paul made some dubious warnings about drone strikes on San Francisco cafes in his filibuster last week against the confirmation of John Brennan as CIA director. But his larger argument for clearer limits on drones is absolutely right.

Paul’s battle shouldn’t have been with Brennan, who said that as CIA director he wouldn’t have any legal power to authorize domestic drone strikes. Brennan, who has been trying to sort out legal rules for drone warfare, deserved to be confirmed, as he was Thursday. Nor was this battle simply with Attorney General Eric Holder, though he did use troubling language in responding to Paul’s queries.

Really, this is an argument with America’s conscience over what actions should be permissible as the war against al-Qaida moves into a new phase. The law can inform this debate about targeted killing, but as with the debate over torture, political leaders must decide that even if action might be technically legal, or effective, it’s morally wrong and therefore impermissible.

We forget how much we’ve debased these ethical standards in the decade since America went to war against al-Qaida and drones became the weapon of choice. I was reminded of this relaxation of our moral code by a former CIA officer who has been involved in many operations where lethal force was used.

The CIA veteran recalled an admonition from a top official in the early 1980s, when the agency began taking lethal actions against Soviet proxy forces: “Never do anything in the field that you know in your heart will shock the conscience of the nation when it becomes public.” That’s still a good rule.

During the early battles against terrorism in the 1980s, lethal operations were truly “capture or kill,” with a strong preference for capture. The CIA’s Counterterrorism Center worked closely with the FBI back then to secretly indict terrorists who had killed Americans overseas. It was understood that the targets might be killed, but the phrase “bring them to justice” wasn’t just a euphemism.

In one case, the CIA had planned to capture a notorious Palestinian bombmaker in a North African country. The operation was scratched at the last minute because of fears it would embarrass the country where it took place.

Under President George W. Bush, the limits on targeted killing were clear, initially. In 2002, the president had to give the CIA special new authority to target senior Iraqi commanders in the run-up to war. After the Iraqi surrender, many of the notorious “deck of cards” leaders were captured, not killed — including Saddam Hussein himself.

Drones changed the balance. They provided a new technology of retribution and became an almost addictive way of using lethal force without risking U.S. casualties. Drones do not capture; they kill. That’s why it’s so important to have clear rules. 

Holder was so determined to keep presidential options open that he gave a vague answer to Paul’s question about targeting U.S. citizens on American soil. “It is possible, I suppose, to imagine an extraordinary circumstance in which it would be necessary and appropriate,” he said — conjuring up another Pearl Harbor or 9/11 when the suicide attackers were Americans. That answer worried millions of other Americans who fear abuse of government power.

Holder’s Justice Department also left too much wiggle room in its recent “white paper” on killing U.S. citizens abroad. The paper argued that such killing is lawful if the citizen poses an “imminent threat” and “capture is infeasible.” But it then fuzzes these standards by saying “imminence” doesn’t require “clear evidence” of an attack “in the immediate future.” As for capture, it can be waived because of “feasibility,” “undue risk to U.S. personnel” or foreign objections.

Sen. Dianne Feinstein has suggested that the U.S. needs a “drone court” to approve targeted killing. But Garrett Epps, a law professor at the University of Baltimore, countered persuasively in The Atlantic online that using courts for such executive action would be unconstitutional. Epps noted the alternative possibility that drone attacks be reviewed by the President’s Intelligence Advisory Board, an independent, bipartisan executive body.

These questions are urgent, as al-Qaida morphs into smaller spinoffs and the existing legal justification for attacks becomes more tenuous. The Washington Post reported last week that to attack al-Qaida offshoots in such places as Syria, Libya and Mali, the U.S. might expand targeting to what a source called “associates of associates.”

Drones should be a weapon of last resort. A positive sign was the report Thursday that Osama bin Laden’s son-in-law, Sulaiman Abu Ghaith, has been captured, not killed.