It's hard to answer the question of whether we should limit the use of drones to active, armed conflicts without invoking Humpty Dumpty. In Lewis Carroll'sThrough The Looking Glass, Alice challenges Humpty Dumpty's definition of the word "glory." The ensuing exchange has long been beloved by both linguists and lawyers:
"When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean -- neither more nor less."
"The question is," said Alice, "whether you can make words mean so many different things."
"The question is," said Humpty Dumpty, "which is to be master -- that's all."
Humpty Dumpty knew that arguments about the meaning of words have as much to do with power as with dictionaries, and so it is with drone strikes. Would limiting their use to "active, armed conflicts and for known terrorists" make drone strikes "Acceptable"? Well... it depends what you mean by "active, armed conflicts," what you mean by "known," what you mean by "terrorists," and what you mean by "acceptable."
Here's the basic problem. Under international law and U.S. law, there are different rules for armed conflicts than for ordinary, peacetime situations. To put it starkly, in peacetime, you can't just go around killing people. Try it! (No, don't try it, not really). Just consider the likely outcome if you wander outside and bash a passerby over the head with a brick. You know what will happen, right? You'll be arrested and charged with murder. You could try saying to the arresting officer, "But officer, the guy I killed was my enemy," but odds are this will just add a mental health evaluation to your woes.
During an armed conflict, though, the rules are different. If you're a combatant, and you see someone approaching who you believe might be an enemy, you get to shoot first and ask questions later. In wartime, soldiers have what's called "combatant immunity:" they don't get prosecuted as murderers for killing other people, provided that their lethal acts are consistent with general law of war principles (these include the principles of distinction, necessity, and proportionality).
I'm radically oversimplifying, but you get the basic idea: many things that are unlawful in the absence of an active armed conflict are lawful during an armed conflict, and this applies to drone strikes just as it applies to the use of lethal force via grenade, gun, or slingshot.
So, can you lob a grenade -- or fire a Hellfire missile from a Predator drone -- into a building full of sleeping people? Generally yes, if there's an armed conflict, you're a combatant, and you reasonably believe the building is occupied by enemy soldiers. Generally no, if there's no armed conflict. The police, for instance, can't just decide to blow up a house in which suspected felons lie sleeping.
So a great deal hinges on whether or not we classify something as an armed conflict -- and this is where Humpty Dumpty comes in. The war in Afghanistan is clearly an armed conflict -- no one disagrees with that premise. But is the U.S. in an "armed conflict" with militants in Pakistan, or suspected Al-Qaeda associates in Yemen, or members of the Al Shabaab organization in Somalia?
The Obama administration, like the Bush administration before it, thinks the answer is yes. Their position is that an armed conflict can exist between a state and one or more non-state entities, even if those non-state entities are not publicly identified, their membership criteria cannot be clearly defined, and their activities are geographically dispersed.
Many others disagree. In Europe, for instance, as a recent European Council on Foreign Relations report by Anthony Dworkin notes, most legal scholars and courts "[reject] the notion of a de-territorialized global armed conflict between the U.S. and Al-Qaeda," and believe that a "confrontation between a state and a non-state group only rises to the level of an armed conflict if the non-state group meets a threshold for organization... there are intense hostilities between the two parties... [and] fighting [is] concentrated within a specific zone (or zones) of hostilities."
Return to the original question: " Should we limit our use of drones to active, armed conflicts and for known terrorists? And would they be acceptable in that case?" Virtually everyone would agree that drone strikes are permissible during active armed conflicts, as long as they otherwise comply with the laws of war (by not targeting civilians, etc.) That's the easy answer. The problem is that we can't seem to agree on what constitutes an "armed conflict" -- and if we can't agree on that, we can't agree on what rules apply, or whether drone strikes are lawful wartime activities or unlawful murders.
The same goes for the concept of "known terrorists." What does it mean to be a "terrorist"? Everyone knows the old quip, famously cited by Ronald Reagan: "One man's terrorist is another man's freedom fighter." More to the point, one man's "Al Qaeda associate" is another man's civilian: there's little agreement on who constitutes a "combatant" in the context of terrorism, or what counts as "direct participation in hostilities." As a result, there's little international consensus on who can lawfully be targeted with lethal force, even if we can agree that an armed conflict exists. An Al Qaeda operative with bombs strapped to his chest? Sure. But what about the AQ financial supporter, or the propagandist, or their wives, or Somali militants whose operations are confined to Somalia?
And what is a "known" terrorist? "Known" by whom? What standards of evidence do we use? Is it enough if secret intelligence, never made public, indicates to unnamed U.S. officials that someone is a probably terrorist, or is it necessary to have a higher level of certainty, or some sort of external review of the evidence? (To quote another U.S. President, Bill Clinton -- a man Humpty Dumpty would surely have admired -- "It depends on what the meaning of the word 'is' is.")
In the end, deciding whether drone strikes are "acceptable" isn't really a legal question, but a question about what kind of world we want to live in, and what restraints we believe should be imposed on those with power. Do we want to live in a world in which the U.S. alone defines who can be targeted and killed in a battlefield with no borders?
Are we comfortable living in Humpty Dumpty's world?