Over the course of the twentieth century, democracies demonstrated an uncanny ability to win wars when their survival was at stake. As this book makes clear, this success cannot be explained merely by superior military equipment or a particular geographical advantage. Instead, it is argued that the legal frameworks imbedded in democratic societies offered them a fundamental advantage over their more politically restricted rivals. For democracies fight wars aided by codes of behaviour shaped by their laws, customs and treaties that reflect the wider values of their society. This means that voters and the public can influence the decision to wage and sustain war. Thus, a precarious balance between government, parliament and military leadership is the backbone of any democracy at war, and the key to success or failure. Beginning with the sixteenth- and seventeenth-century writings of Alberico Gentili and Hugo Grotius, this book traces the rise of legal concepts of war between states. It argues that the ideas and theories set out by the likes of Gentili and Grotius were to provide the bedrock of western democratic thinking in wartime. The book then moves on to look in detail at the two World Wars of the twentieth century and how legal thinking adapted itself to the realities of industrial and total war. In particular it focuses upon the impact of differing political ideologies on the conduct of war, and how combatant nations were frequently forced to challenge core beliefs and values in order to win. Through a combination of history and legal philosophy, this book contributes to a better understanding of democratic government when it is most severely tested at war. The ideas and concepts addressed will resonate, both with those studying the past, and current events.