States commit to military alliances for numerous reasons. Consensus on many variables among the states in these alliances is a key factor in why they form alliances and establish written treaties. However, not all military cooperation results in a written treaty, even when facing an external threat. This study explores the role of consensus on external threats as an explanation for why some states formalize their military cooperation in a written treaty while others do not. Using a case study comparing the Nato-alliance and the Quad-cooperation, this study uncovers how the ideological threat, coupled with shared state security interests, serves as an explanatory factor. This insight has implications at the strategic military level, helping to define common security interests among states and foster consensus on dealing with external threats, ultimately leading to the establishment of written treaties.