Chris Bertram has been researching the legal basis for drone strikes:
As far as I can see, the strikes need to pass the regular tests of discriminating between combatants and non-combatants and not causing disproportionate “collateral damage”. They also need to get past the UN Charter’s ban on using force against the territorial integrity of other states. This article by Jack Goldsmithclaims there are two ways to do this (1) by getting consent from the “victim state” and (2) by properly invoking the right of self-defence re the non-state actors concerned and claiming truly that the victim state is “unwilling or unable” to deal with the threat posed.
He states that drone strikes are violating international law
The United States in its drone campaign appears to be relying on self-defence and this “unwilling or unable” test. This strikes me as deeply problematic on two grounds. The first is that the Caroline test, that necessity of self-defence be “instant, overwhelming, leaving no choice of means, and no moment of deliberation” seems not to be met. But I suppose the United States could claim that it is simply continuing a campaign of self-defence that began after September 11th 2001 and has continued since. (Could that really justify extending “self-defence” to take in new sovereign territories?) The second reason is that it looks to me as if the doctrine the United States is relying upon would also have justified “targeted assassinations” by other states on US soil at various times against individuals or groups planning or engaged in actions against those states, whom the US was unwilling to suporess. So, for example, both Cuba and Nicaragua in the past and maybe Iran today could invoke a similar doctrine with as much justification. Say it ain’t so?
Moral issues aside, these drones strikes are simply violating International law.