Over the last few years, the military landscape has undergone considerable modification. Not only are we witnessing changes with regard to the adversaries that fight one another (consider the rise of what has been labeled "asymmetric warfare"), but the means by which armed conflict is carried out has undergone significant modification with more--and potentially more transformative--changes yet to come.
The most obvious of these changes is already underway--and has come under some scrutiny. So-called unmanned aerial vehicles (UAVs) have conducted a vast and increasing number of reconnaissance missions. A smaller number of missions carry out armed attacks, with the operators of either type of mission connecting to their aircraft via satellite link from thousands of miles away. Similar attempts have been made by militaries around the world regarding sea and land warfare. All of these examples--whether operating by air, sea, or land--share one characteristic. These unmanned systems (UMS) are the visible pieces of a network that (at least until this point) still operates with direct human input.
Building on these first steps toward greater autonomy for weapon systems, the next generations of UMS are designed to operate wholly independently from human input. From target selection, to the decision whether to employ weapon systems in the particular moment in time (and, if so, which weapons), autonomous weapon systems (AWS) will be in a position to carry out their missions without direct human input. This development changes the assumptions on which today's jus in bello is based and has the potential to alter fundamentally the perceptions of that field of international law.
It is important to distinguish AWS from already existing technologies such as remotely operated systems or automated systems. Remotely operated systems have been in place for some time (with early examples going back to the end of the 19th century) and have received increased attention in recent years with the increase of attacks carried out by UAVs, including so-called targeted killings. (1) Automated weapons have been employed regularly over the last half century, one example being the use of cruise missiles. (2) AWS differ from these weapons. Unlike remotely operated systems and automated systems, AWS do not require a human operator to be in the loop. (3) Rather, by design they operate without direct human input. (4)
This development raises legal, ethical, and political concerns. The challenge that AWS pose concerns their compatibility with today's jus in bello. In this context, two cornerstones of international humanitarian law are of particular relevance: the principle of distinction and the principle of proportionality. (5) Both principles are designed to protect civilians, but also allow for attacks, provided that the military advantage gained by the attack (as anticipated at the time of the attack) does not disproportionately harm the civilian population.
The principle of distinction--spelled out in Article 48 of Additional Protocol I to the 1949 Geneva Conventions and refined in subsequent provisions--mandates that when carrying out an attack, combatants must distinguish between combatants (as well as military objects) and civilians (as well as civilian objects). Additional Protocol I not only prohibits attacks that target civilians only or that target purely civilian objects, but also bans the use of weapons that are by their nature indiscriminate. (6) These limitations become ever more important in a battlespace (as opposed to the conventional battlefield) in which civilian and military targets become increasingly intertwined.
Some of the determinations with respect to the principle of distinction can be described quantitatively. While relying on largely quantitative analysis may be possible for some situations (imagine a tank which has specific characteristics that make it distinguishable from a car or a bus), others--especially those involving changing or unclear circumstances--are less amenable to mechanistic analysis (imagine a person carrying a tool over his shoulder that looks very similar to a weapon). …