jats:titleAbstract</jats:title>jats:pWhy have states created international laws to regulate internal armed conflicts? This article is the first to theorize the emergence and design of these international rules, focusing on Common Article 3 to the 1949 Geneva Conventions. Drawing on original multicountry archival research, I develop the mechanism ofjats:italicforum isolation</jats:italic>to explain the origins of Common Article 3, demonstrating the importance of social opprobrium pressure to explain why Britain and France switched from staunch opposition to support and leadership in 1949. Specifically, forum isolation pressured these European empires to concedejats:italicand</jats:italic>to react strategically behind the scenes, saving face and safeguarding their security interests by deliberately inserting ambiguous language in the text of Common Article 3. This move later facilitated states' avoidance of this rule in many conflict cases.</jats:p>