Humanitarian Law applies when accepted rules of engagement in war have not been respected.
A primary concern of Humanitarian Law is to make sure that wars follow specific rules of engagement. These are standards that justify the use of force in war. It's difficult to imagine that something as brutal as war has "rules." However, the rules of engagement that military forces follow are "rules" that govern soldiers' conduct.
Humanitarian Law is concerned with ensuring that war does not move into cruelty: "It [Humanitarian Law] comprises a set of rules, established by treaty or custom, that seeks to protect persons and property/objects that are (or may be) affected by armed conflict and limits the rights of parties to a conflict to use methods and means of warfare of their choice." Humanitarian Law applies when armed conflict moves past its limits and causes undue human suffering.
Humanitarian Law affirms the idea that war is not a license to suspend civil and political rights:
The Court observes that the protection of the International Covenant of Civil and Political Rights does not cease in times of war, except by operation of Article 4 of the Covenant whereby certain provisions may be derogated from in a time of national emergency.
War moves into unspeakable domains when atrocities are committed against civilians. Humanitarian Law applies when these situations constitute being far beyond the standard interpretation of war.
For example, International Law applies in the Congo, where sexual violations of women are rampant. In Afghanistan, the actions of the Taliban can also be seen as an instance where International Law is applicable. An interesting issue that is emerging involves the United States and its use of drone technology in fighting its war on terror. As drone use increases and it is becoming clear that civilians are dying as a result of drone use, questions are being raised as to whether or not International Law is applicable. In each of these situations, International Law applies when the traditional understanding of war's rules of engagement are violated. To minimize undue human suffering becomes a foundation of Humanitarian Law.