Chapter 4 considers evidence that disagreements about the jus ad bellum are linked to disagreements between ‘formalist’ and ‘dynamist’ legal cultures. The chapter describes legal interpretation techniques identified in analysis of UK government statements and writings by eight legal scholars about the lawfulness of military action in Kosovo (1999), Afghanistan (2001) and Iraq (2003), judges’ opinions in the key ICJ cases Nicaragua (1986), Wall (2004) and Congo (2005), and in interviews and a survey with thirty-one UK-based international lawyers. The analysis suggests the jus ad bellum displays the forms of vagueness already identified: paradigms, supervaluationism, and fuzzy logic. Lawyers’ legal interpretive choices broadly aligned with their views on the lawfulness of force, on a continuum between a formalist legal culture aligning with a restrictive approach to the jus ad bellum, and a dynamist culture aligning with an expansionist approach. But the correlation has caveats: expansionist lawyers sometimes deployed formalist arguments, while restrictivist lawyers sometimes deployed dynamist arguments. Competing interpretation techniques also do not explain lawyers’ differing factual assessments and forecasts about the jus ad bellum.