Following Russia’s annexation of Crimea in 2014, the recent conflicts in Ukraine and the Middle East signal the return of geopolitics. This book challenges conventional approaches that ignore border change, arguing that geopolitics is driven by nationalism and focusing on how nationalism transforms the state. Using geocoded historical maps covering state borders and ethnic groups in Europe, the authors’ spatial approach shows how, since the French Revolution, nationalism has caused increasing congruence between state and national borders and how a lack of congruence increased the risk of armed conflict. This macro process is traced from early modern Europe and widens the geographic scope to the entire world in the mid twentieth century. The analysis shows that the risk of conflict may be increased by how nationalists, seeking to revive past golden ages and restore their nations’ prestige, respond to incongruent borders. This title is also available as Open Access on Cambridge Core.