Proudhon’s anarchist theory of international relations, a 19th-century vision of what might have been and could still be. War and Peace by Pierre-Joseph Proudhon, originally published in 1861, is still one of the only extended accounts of anarchist international theory and is one of the earliest in the history of socialist thought. It is a profound contribution to the traditions of jus gentium and just war theory, that puts force and power at the centre of analysis. Alex Prichard’s introduction describes both its specificity and the multiple lines of influence War and Peace had on thinkers as diverse as Tolstoy, Sorel, French sociology more broadly, and post-1945 Anglo-American International Relations theory.