Principle of distinction is one of the cardinal principles that constitute the fabric of humanitarian law. Military attacks are authorised against military objectives, combatants and members of armed forces as well as against civilians who directly participating in hostilities (so called legitimate targets). The purpose of this principle is to restrict warfare to acts of violence against the enemy combatants. The civilian (and its objectives) shall be spared against the effects of hostilities. Conversely, the combatants who directly participate in conduct of hostilities are legitimate military target at all times except when they are rendered hors de combat.
Additional Protocol provides definitions of all legitimate targets. In case of ambiguity, it should be presumed that the object is being used for civilian purpose. The belligerents must take precautionary measures to limit negative effect of the attack. These measures oblige parties to take actions in order to protect civilian population against the effect of military attack. Belligerents should not locate military objective within or near densely populated areas or envisage relocation of civilians in order to protect them.
Art 50 of the AP1 provides a negative definition of civilians, probably the most important definition of international humanitarian law. Civilians are not members of the military, armed groups and in event of any doubt; the law requires presumption that a person is a member of civilian population. Civilians cannot be object of attack unless they directly participate in hostilities. Interpretative Guidance (2009) of ICRC clarifies the term of direct participation, which requires a direct relationship to host tiles, temporal or geographical proximity and causation between act of civilian and harm resulted from this act.
Principle of distinction also provides special category of protection to medical units, prisoners of war, and installations containing dangerous forces (dams, nuclear power plants), cultural and historical objects and natural environment. These categories shall not be the object of any military attack.
Despite the challenges modern armed conflicts pose to principle of distinction, it still remains a significant humanitarian principle, which protects civilians and their objects.