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.