Chapter 5 considers evidence that disagreements about the jus ad bellum are linked to disagreements about between ‘pacificist’ and ‘interventionist’ strategic cultures ‘extra-legal’ politico-strategic and ethical principles. The chapter describes extra-legal reasoning, particularly in evaluating facts, in 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 ICJ cases Nicaragua (1986), Wall (2004), and Congo (2005), and in interviews and a survey with thirty-one UK-based international lawyers. The chapter concludes that lawyers’ extra-legal reasoning and views on lawfulness of force broadly align, on a continuum between pacificists preferring a restrictive jus ad bellum, and interventionists favouring an expansionist approach. But again, there are caveats. Most interventionists accept some legal prohibitions they believe are politically or ethically wrong. Most pacificists accept some justifications they politically or ethically condemn. This suggests most lawyers’ politico-strategic and ethical intuitions act as forms of cognitive biases, shaping but not wholly determining opinions about legal interpretation and the jus ad bellum.

Chapter 3 considers factual uncertainty in the jus ad bellum. It describes how the vagueness described in Chapter 2 can affect factual evidence in the jus ad bellum. Such situations are often chaotic, fast moving, involving actors trying to mislead, and where factual evidence is open to different interpretations. Tests such as necessity, imminence and proportionality require decision-makers to compare multiple forecasts of outcomes if force is used or not. Survey participants evaluating four fictional conflict scenarios, demonstrating how even fictional ‘facts’ elicit varying opinions. The ICJ’s rules of evidence only reduce rather than resolve factual uncertainty. The chapter suggests a second potential explanation for contestation in the jus ad bellum: lawyers resolve factual uncertainties and make forecasts using their politico-strategic and ethical intuitions, forming competing strategic cultures disagreeing about legality of specific cases and the jus ad bellum more generally. Thus ‘restrictivist’ lawyers prefer a ‘pacificist’ strategic culture, seeing little extra-legal justification for force, while ‘expansionist’ lawyers prefer an ‘interventionist’ strategic culture, seeing more such extra-legal justifications.