For peace advocates a corollary to Clausewitz’s dictum that “war is politics by other means” might be that other politics could prevent war. By highlighting both individual peace advocates and antiwar/peace organizations from World War I through the wars of the 21st century, the chapters will provide insights into how these individuals and organizations articulated their opposition to and mobilized against specific wars and international/regional conflicts. Organized roughly in chronological order, each chapter will illuminate the socio-historical conditions under which such peace advocacy contested state aggression and armed combat at the national and/or transnational levels. Beyond understanding the specific socio-historical circumstances within which these antiwar and peace advocates and organizations operated and their resultant achievements and failures, the book as a whole will examine the kind of politics that perpetuate war and those that offer a challenge to that perpetuation. Scholars, students, and the general public interested in the history of modern and contemporary wars, peace and conflict studies, and ethical/political perspectives in the 20th and 21st centuries should find much to reflect upon in this book.