This chapter provides context for the study as a whole, situating the development of the civil war regime within the evolution of the international law of armed conflict. The chapter considers the classical, mediaeval and enlightenment manifestations of the legal distinction between international and internal armed conflict. It then discusses the rise and fall of the doctrines of belligerency and insurgency, the Lieber Code, and the establishment and early activities of the ICRC, before surveying the development of the law of armed conflict from the late 19th century until the Second World War. The chapter then considers the development of the civil war regime between 1949 and 1998 and subsequently, highlighting the key elements of the regime and setting these in the broader international legal context, including the development of international human rights law and international criminal law.