"There are few concepts in international law that captivate the mind and cause bitter debates as much as the application of the principle of proportionality in contemporary armed conflict. As conflicts are increasingly fought in urban settings, the concept of proportionality plays a crucial role in the protection of civilians from the horrors of war. This book is a study of proportionality as it is currently understood in the laws of armed conflict-also known as international humanitarian law (IHL). The principle of proportionality is one of the cornerstones of IHL, together with the other basic principles of distinction between civilians and combatants, the prohibition on the infliction of unnecessary suffering, the notion of military necessity, and the principle of humanity. Although proportionality is notoriously opaque and can seem a topic for theoreticians, the real-world impact of this principle and its interpretation is very concrete. States, and their armed forces, treat it as a serious limitation on their military activity. There are many reasons for the prominence of the principle of proportionality, especially, as Thomas Franck explained, its hold on the "imagination of the epistemic community in which it is used as the prism for viewing, arguing, and ultimately resolving disputes." It is, perhaps, useful to present the principle of proportionality in the context of two other cardinal principles of IHL: the principles of necessity and of distinction. According to these, only combatants and military objectives may be targeted during armed conflicts. The attacking party must ascertain whether a given target is military or civilian, and refrain from attacking the latter. The principle of proportionality adds a further constraint to the principle of distinction. Even if the target is a military objective, attacking it is prohibited if it is expected to cause incidental harm to civilians "which would be excessive in relation to the concrete and direct military advantage anticipated." For example, while civilian buildings cannot be attacked during a military operation, a building which is housing a group of soldiers can be attacked, because this makes it a military objective. However, the principle of proportionality place limits on how and when this building may be attacked. It is prohibited to attack the building if it is known that a large number of civilians either in the building or in its vicinity would be harmed, even if unintentionally, to an extent that this harm ("collateral damage") would be excessive relative to the military advantage gained by the attack. Thus, distinction and proportionality impose two cumulative conditions which must both be fulfilled in order for an attack to be lawful. Both principles limit the freedom of operation and the discretion of the belligerent parties, and thereby play an important role in protecting civilians from the vicissitudes of armed conflict"--