"I was motivated to write this book by my own perhaps naïve surprise, as an official in the UK's Foreign, Commonwealth and Development Office (FCDO), coming to the realisation that even respected international lawyers had fundamental disagreements about the legality of the US-led military interventions in Kosovo in 1999, Afghanistan in 2001 and of course Iraq in 2003. Like many FCDO officials, I had a basic understanding of international law and experience of working with the FCDO's international lawyers. So I had some sense that international law had 'grey areas', and that plausible legal arguments could often be advanced for opposing positions. However, I assumed even difficult legal questions ultimately always had a single correct answer, which the FCDO's lawyers could be relied upon to discover after sufficient study of doctrine, precedent and the relevant legal materials. Underpinning this assumption was a sense that such legal questions would ultimately be decided by some authoritative tribunal or other dispute resolution mechanism. It was the sharp and ultimately formally unresolved debates between respected international lawyers around the legality of use of force in Kosovo, Afghanistan, and Iraq which led me to realise the naivety of this assumption, and to seek to deepen my knowledge of international law.This book is the first which seeks to synthesise approaches from these different disciplines to offer new ways of understanding and dealing with uncertainty, controversy and the role of extra-legal intuitions in hard cases engaging international law governing resort to military force. Unlike other studies of the jus ad bellum, this book does not try to identify what the law is, nor to prescribe 'the' 'correct' method for framing and assessing legal arguments. Rather, this book explores how legal reasoning works in this area of law, using concepts from the philosophy of knowledge to explain what it is about the jus ad bellum which enables uncertainty and disagreement. This book casts light on why and how lawyers' political, ethical and strategic intuitions about how the world works and how it ought to work shape their legal assessments of hard cases engaging this law. The book considers how uncertainty about current and future facts feeds into legal uncertainty - how hard cases of force often require complex factual assessments, and forecasting of the immediate and long-term consequences of both using and not using force"-
