Is the world destined to suffer endless cycles of conflict and war? Can rival nations become partners and establish a lasting and stable peace? How Enemies Become Friends provides a bold and innovative account of how nations escape geopolitical competition and replace hostility with friendship. Through compelling analysis and rich historical examples that span the globe and range from the thirteenth century through the present, foreign policy expert Charles Kupchan explores how adversaries can transform enmity into amity--and he exposes prevalent myths about the causes of peace. Kupchan contends that diplomatic engagement with rivals, far from being appeasement, is critical to rapprochement between adversaries. Diplomacy, not economic interdependence, is the currency of peace; concessions and strategic accommodation promote the mutual trust needed to build an international society. The nature of regimes matters much less than commonly thought: countries, including the United States, should deal with other states based on their foreign policy behavior rather than on whether they are democracies. Kupchan demonstrates that similar social orders and similar ethnicities, races, or religions help nations achieve stable peace. He considers many historical successes and failures, including the onset of friendship between the United States and Great Britain in the early twentieth century, the Concert of Europe, which preserved peace after 1815 but collapsed following revolutions in 1848, and the remarkably close partnership of the Soviet Union and China in the 1950s, which descended into open rivalry by the 1960s. In a world where conflict among nations seems inescapable, How Enemies Become Friends offers critical insights for building lasting peace.