Originally promulgated in the time of kings when wars of aggression were the sovereign's prerogative, international custom and later treaties prohibiting attacks on the leader arose from kings' mutual desire to protect themselves. In the post-League of Nations and UN Charter era, aggressive war is illegal under international law, but many dictatorial rulers and non-state commanders have continued to benefit from the prohibition on assassination traditionally afforded to kings.
Black's Law Dictionary defines assassination as "the act of deliberately killing someone especially a public figure, usually for hire or for political reasons." If termed "assassination," then attacks on leaders have been construed as prohibited by Article 23b of the Hague Convention of 1899, which outlaws "treacherous" attacks on adversaries, and by the Protocol Addition to the Geneva Convention of 1949, and Relating to the Protection of Victims of International Armed Conflict (Protocol I), which prohibits attacks that rely on "perfidy." But in recent years, and especially since the September 11, 2001, terrorist attacks, Israel and the United States have reframed such actions as "targeted killings," defining the victims as "enemy combatants" who are therefore legitimate targets wherever they are found. This redefinition has relied on and benefited from the work of some in the international law community who have long argued that in some instances, targeted attacks on leaders are not prohibited by international law. This reinterpretation of law is not a radical shift; the radical shift is US and Israeli willingness to engage in attacks openly, whatever may have occurred covertly in the past decades. Strong pragmatic reasons, such as sparing the lives of troops who would be killed in a large scale assault, justify targeting leaders if possible, but such a policy opens the employing country to reciprocal attacks, justified or not, on its own leaders. In some cases, killing militant leaders may do more harm than good by further inflaming an already tense situation and causing retributory attacks. Killing adversary leaders can fall within the bounds of international law and can provide enormous gains, but in employing this strategy, the United States and countries that follow its example must be prepared to accept the exploitation of the new policy by adversaries who will not abide by the standards of proof or evidential certainty adhered to by Western democracies.
As with many areas of international law, the status of assassinations is relatively ambiguous. The clauses that traditionally have been construed as prohibiting "targeted killings" are far from clear prohibitions. In the Hague Convention (II) with Respect to the Laws and Customs of War on Land (29 July 1899), Article 23b states that it is prohibited "to kill or wound treacherously individuals belonging to the hostile nation or army." Treachery is not explicitly defined, and it can be argued that using missiles to attack a car in which a target is traveling, while brutal and having a high probability of injuring bystanders, does not fall within the purview of treachery. Similarly, targeted killings can be argued to fall outside the Protocol I Article 37 prohibition on killing, injuring, or capturing "an adversary by resort to perfidy"--described as "acts inviting the confidence of an adversary to lead him to believe that he is entitled to, or is obliged to accord, protection under the rules of international law applicable in armed conflict, with intent to betray that confidence." Article 37 gives examples of perfidy including "the feigning of an intent to negotiate under a flag of truce or surrender" and "the feigning of civilian, non-combatant status." The manner in which Israel and the United States have engaged in targeted killings does not meet this definition of perfidy because neither state leads targets to believe that they are protected by international law. …