This dissertation investigates the political origins of the international rules for internal armed conflicts. It explores the following questions: Where do these norms and rules come from? Why have states negotiated and accepted them? How and why have international legal responsibilities been assigned to armed non-state actors waging internal wars? The two crucial international instruments of this type are Common Article 3 to the Geneva Conventions of 1949 and the Additional Protocols to those Conventions, from 1977. Through them states agreed to limit their means of action via binding treaty law while promising to extend humanitarian care to their challengers, at the risk of favoring them militarily or of legitimating their cause. States also extensively debated whether and how their armed contenders should bear responsibility. These are historically important and theoretically counterintuitive developments in international law and security, and this is the first work of political science to investigate them on the basis of extensive multi-archival research (amounting to over 35,000 declassified documents) and interviews conducted in four countries (the UK, the US, France and Switzerland.) I adopt the qualitative methods of process-tracing and focused case comparisons to answer the research questions. The argument unveils in two phases. In a first stage, I show how international shocks, or domestic shocks of international proportions (typically, major civil wars) progressively opened windows of opportunity which 1) evinced the "need to do something" about internal conflicts; 2) motivated prominent non-state actors to take up the issue and press for change; and 3) facilitated states' acceptance to work out new international rules either by morally motivating some of them or by helping others relax their initial reluctance toward them. Yet I argue also that socializing "the need to do something" was only half the work. Between the collapse of the old orthodoxies and the construction of new norms, much politics occurred. I theorize this second stage as one in which states waged struggles in international conferences to hammer out consensus formulae, with different groups trying to influence/coerce their opponents into accepting their vision. In 1949, for instance, while several Western democracies (including the United States, Scandinavian and Latin American countries) accepted the idea of the humanitarian regulation of civil wars, many others still fought hard to include high restrictions on its application. The Soviet Union fiercely supported the idea against all predictions, while the United Kingdom and France, already facing turmoil in their colonies, strongly opposed them and tried to warn their peers of the dangers posed by rules that might encourage or protect rebels. In the midst of these various pressures, I show how the pro-regulation majorities in the room managed to socially coerce (rather than persuade) skeptics into accepting the idea of making new rules. In the 1970s, under the powerful banner of self-determination, a majority coalition of newly-independent, Third World and socialist states forced the diplomatic hand of otherwise more powerful Western states into accepting polemical language that legally legitimated "armed conflicts in which peoples are fighting against colonial domination and alien occupation and against racist regimes" as international conflicts. Simultaneously, this coalition fought hard to water down standards to regulate other types of internal conflicts, which many of them were experiencing in the aftermath of decolonization. In a final step, having shown the effects of social coercion in diplomatic negotiations, I also detail how coerced states reacted or "pushed back" covertly by crafting counterproposals whose language seemed acceptable to them at the same time that it appeared to address the concerns of pro-regulation groups. In the case of Common Article 3 of 1949, I detail how the UK and France jointly re-shaped the resulting text to read simultaneously generous and vague, knowing this might allow them to "interpret their way out" of the commitment later on. In the 1970s, Western states swallowed the bitter pill of enshrining violent struggles for self-determination in international law while making sure the language applied only to specific and increasingly rare conflict situations. Far from representing a persuasive consensus, I argue that final regulatory outcomes in this issue-area are difficult compromises that strike an uneasy balance between various' states' concerns for status legitimacy, morality and military expediency, whose conflicted political origins partly explain their limited impact in practice.