When does the application of international humanitarian law properly begin and end in modern conflicts? Classical international law distinguished three types of armed conflict: (1) war; (2) civil war; and (3) armed hostilities short of war. The laws of war were applicable in time of war--from the declaration of war until the formal reestablishment of peace (for example, by the signing of a peace treaty). The laws of war were not applicable in civil wars--which were considered internal matters--unless a state formally recognized the insurgency as a belligerent. And, of course, the laws of war were not applicable as a formal matter in hostilities short of war. Prior to the drafting of the Geneva Conventions in 1949, the applicability of the “law of war” was, therefore, delimited by formal acts of state such as a formal declaration of war or a formal recognition of belligerency. The Geneva Conventions substantially revised this formalistic, de jure approach--making contemporary international humanitarian law applicable during armed hostilities that de facto constitute “armed conflicts.” In both international and non-international armed conflicts, the Geneva Conventions, in general, govern the conduct of hostilities for the duration of the “armed conflict.” This background note briefly outlines the regime established in the Geneva Conventions and summarizes several ambiguities in these rules. Because the scope of application regimes differ sharply between international and non-international armed conflict, these two types of conflict are analyzed separately.