This chapter examines the historical development and current status of international law aimed at restricting the resort to the use of armed force in relations between nations. Relying on natural law, Christian theologians drew no distinction between morality and legality. The First World War lasted four years. The war caused the political, economic, and human ruin of Europe. As the postCold War era has worn on, peacekeeping has taken on a more expansive meaning, morphing frequently into peace enforcement and peace building. As with peacekeeping, these terms are not mentioned in the United Nations Charter. Traditionally, international law has been primarily concerned with international armed conflict, not non-international armed conflict. Civil wars have fallen into the latter category and have been on the periphery of international law. It has been noted that foreign involvement in civil wars in the postCold War era frequently has taken the form of humanitarian intervention.