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 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.

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.

Chapter 2 considers legal uncertainty in the jus ad bellum as defined in the UN Charter and other sources of law. It describes paradigms, framing this law around ‘plain cases’ of lawful and unlawful force. Supervaluationism describes paradigms as determined not by one but several tests, overlapping but not co-extensive - cases may meet all or only some tests. Fuzzy logic refutes binary distinctions between lawful and unlawful force, arguing these are end-points of a continuum, separated by a ‘penumbra of uncertainty’. The chapter outlines ‘hard cases’ of force, different to cases the UN Charter most obviously prohibits: anticipatory self-defence, pre-emptive self-defence, self-defence against non-state actors, humanitarian intervention, use of force to prevent WMD proliferation. The chapter describes how interviewees and survey participants evaluated such justifications, the results displaying the vagueness already identified. The chapter identifies one possible explanation: lawyers align with different ‘interpretive cultures’, holding different opinions about valid legal tests and interpretive techniques. ‘Restrictivists’ prefer ‘formalist’ interpretation techniques, while ‘expansionists’ prefer ‘dynamist’ interpretation techniques.

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.

Chapter 6 considers how insights from legal risk management, international humanitarian law, strategic intelligence analysis and forecasting might help manage uncertainty and extra-legal biases in the jus ad bellum. The chapter proposes a framework drawing on these fields, using legal and extra-legal expertise to describe the legal and factual context, competing legal theories justifying or prohibiting force, multiple possible interpretations of current facts and future consequences of using or not using force and potential risks that any legal theory is challenged or disproven, and recommending whether to accept these risks and use force, take steps to reduce risks, or not to use force where these risks are too severe. The chapter applies this framework to the Kosovo and Afghanistan interventions, showing that the framework as applied by this author recommends more cautious decisions than those actually taken. The chapter considers how such a framework might have affected UK legal advice on the 2003 Iraq intervention. The chapter concludes that the framework may help address uncertainty and extra-legal intuitions, but does not eliminate the need for judgement, and raises significant normative questions.

Chapter 8 concludes the book by reviewing its arguments and conclusions, identifying weaknesses in the methods used, and proposing ways in which these might be addressed and the research developed further, for example by involving participants in other countries. The chapter importantly notes that uncertainty and contestation remain marginal problems in the jus ad bellum, as in most areas of international law. In most cases, the requirements of international law governing resort to military force are clear and uncontested. There are many reasons to believe that the contemporary jus ad bellum has contributed to global peace and stability. This book’s examination of legal and factual uncertainty and extra-legal intuitions seeks to support and assist lawyers and states in their mission to uphold and apply this crucial area of international law, not to encourage lawyers or states to undermine or abandon it.

Chapter 7 considers how the framework proposed in Chapter 6 potentially contradicts international lawyers’ professional responsibilities. The chapter first considers different approaches to such questions in different countries and professional contexts. It then examines lawyers’ professional responsibilities when dealing with difficult jus ad bellum cases, reporting interviewees’ views of such responsibilities and of risk management techniques. By identifying and reducing risks of legal challenge to use of force, the framework potentially focuses lawyers only on their role as ‘counsellor’, helping governments do whatever they want, rather than their normative role of ‘conscience’, urging governments to change their behaviour to abide by law. Such a framework might even advise decision-makers to accept clearly unlawful force if there is little risk of an action facing legal challenge. The chapter nevertheless argues lawyers could use the framework developed in Chapter 6 while fulfilling their professional responsibilities, by seeking a form of Rawlsian ‘reflective equilibrium’ with their own personal professional judgement to address legal and factual uncertainty and extra-legal intuitions in hard cases engaging the jus ad bellum.