This book makes an original contribution to international relations and British politics. It identifies for the first time the dominant pre-modern theory of international relations, which fatalistically assumed that war was beyond human control. It then shows how this theory was undermined from the 1730s onwards, with the consequence that a debate began about how best to prevent war in which a vocal minority argued that war as an institution for settling disputes could be abolished. Britain led the way in this repudiation of fatalism and exploration of pacific alternatives: it produced the world's first peace movement (which appeared in the mid-1790s as a response to the French wars) and the first enduring national peace association (the Peace Society, founded in 1816 and active for nearly a century); and it was the first country to allow peace thinking (for example, as expounded by Richard Cobden) to enter its political mainstream. This book, the first to make use of the Peace Society's records, fills a major gap in the historiography of British politics.