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 counbtry to allow peace thinking 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.