The importance of World War I for understanding contemporary East Asia lies not in the ubiquitous analogy drawn between Anglo-German antagonism and contemporary U.S.-China relations, but rather in the more specific lessons the period preceding its outbreak offers concerning the sources of instability and conflict among states. More precisely, these lessons relate to the challenges posed by complex security arrangements, the dual-edged nature of nationalism, and the dangerous dynamics that can emerge during repeated crises. Appreciating how these factors contributed to mounting tensions and eventually the onset of war in Europe a century ago can enhance efforts to understand and manage tensions in East Asia today.