Humanitarian Law requires that belligerents engaged in hostilities respect the principle of proportionality, which derives Geneva Conventions, their Additional Protocols as well as customary humanitarian law. A party of conflict (a state or armed group) must respect unconditionally the principle even if they are not a party of international commitments.
Art 54(1) prohibits attacks, methods, and means of combat, which are not directed at a specific military objective. As a armed conflict is an unpredicted and uncalculated course of action (we can not ensure that civilian will never suffer), the principle of proportionality required that suffering of civilians cannot be excessive in relation to the concrete and direct military advantage which is anticipated (Art 54 (2) of AP1). This test of proportionality needs to be conducted each time before the military attack to limit unnecessary suffering among civilian population and also belligerents. St Petersburg Declaration and Hague Convention IV, art 22 prohibits namely use of arms to causes unnecessary suffering among belligerents.