How did global heritage become the new battlespace for 21st-century warfare? Nations and non-state actors alike increasingly use the ruins of the past to leverage legal, political, and territorial competition.At the close of the Second World War, UNESCO began a vast reconstruction program for a war-ravaged world. New world histories were written, museums and archaeological sites were preserved, monuments registered, and cultural exchanges fostered. The North Atlantic Treaty Organization also emerged in the wake of WWII and aspired to "safeguard the freedom, common heritage and civilization of their peoples." In its infancy, NATO looked to UNESCO as a model for building transatlantic bridges through cultural programs and events, including adopting monuments, hosting exhibitions, and producing textbooks. The romanticism of these 20th-century responses to ruins in wartime that linked monuments and memory, heroic salvage, moral uplift, and global civilization has gradually shifted to a more conflictual status in the 21st-century, where the past is increasingly weaponized and targeted, as seen most vividly in Russia's assault on Ukraine. Ruined traces the path of these two intersecting international organizations as they seek to combat the fallout from cultural ruin in their respective missions. Beginning with the infamous Allied destruction of the Benedictine Abby of Monte Cassino in WWII, the book traces the history of how, on the one hand, protecting heritage has become vital for multilateral agencies and governments alike to combat illicit trafficking, terror financing, cultural cleansing, and crimes against humanity. At the same time, the past, particularly the heritage of wars past, is alternatively weaponized by governments to reify and enflame conflict anew. Termed "cultural heritage exploitation," or CHX, such hostile threats to security now constitute the subject of intensifying legal regimes of accountability and claims of cultural cleansing and genocide. Bringing us full circle, some historic places of past trauma that have been excavated and memorialized, including the Western Front, the D-Day Landings, Hiroshima, and the Rwandan genocide sites, are recursively celebrated as World Heritage. Ruined demonstrates how heritage assets^&#8212from Gaza and Jerusalem to Odessa and Kyiv and Preah Vihear in Cambodia^&#8212once positioned as global goods throughout the twentieth century are being transformed into weapons of war in the twenty-first.