Chapter 1 outlines the main issues that the book investigates: the nature of legal and factual uncertainty in 'hard cases' engaging international law governing resort to military force, the jus ad bellum; the role of competing interpretive and strategic cultures in generating and sustaining that uncertainty, by operationalising opposing intuitions about legal interpretation, and 'extra-legal' political, strategic and ethical assessment; and the potential usefulness in dealing with these dilemmas of insights from fields outside this area of law, such as international humanitarian law, legal risk management, and strategic intelligence assessment and forecasting. The chapter describes the research methods used: structured interviews and a survey with thirty-one UK-based international lawyers, and systematic textual analysis of key International Court of Justice cases and scholarly assessments of US-led interventions in Kosovo in 1999, Afghanistan in 2001 and Iraq in 2003. Finally, the chapter outlines the overall structure and argument of the book.