When and why did international law begin to oppose war? For centuries, sovereignty implied the right to wage war. Yet over the past hundred years, a remarkable succession of treaties, courts, and organisations sprang to life that sought to prohibit war. From a fringe ambition, the ideal of ‘peace through law’ became the foundation of international law. This book traces part of this evolution back to the small peace movement of the early nineteenth century, recounting how the earliest organised pacifists built their legal case against war. The stories of this diverse social movement are told from numerous perspectives, and each sheds further light on how ordinary men and women helped lay the groundwork for one of the greatest shifts in legal thinking about peace and war.