This book explores how cities, regions, and sub-state actors engage in international politics as illiberalism and authoritarianism reshape the global order. Moving beyond the traditional association between paradiplomacy and democratic decentralization, the book investigates how subnational diplomacy operates under political constraint, centralization, and regime pressure. Drawing on diverse case studies from Turkey, Hungary, Taiwan, Iran, the United States, and Venezuela, the book introduces new conceptual tools—most notably competitive-disharmony—to capture the varied ways subnational actors cooperate with, resist, bypass, or reinforce central authority. It demonstrates that paradiplomacy is neither inherently democratic nor inherently oppositional; rather, it is a politically contingent practice that can serve regime consolidation, ideological projection, survival, or resistance. By combining conceptual refinement with empirically grounded analysis across multiple regions, the book offers a comparative framework for understanding sub-state diplomacy in an era of democratic backsliding and global fragmentation. This volume will be of interest to scholars and students of International Relations, comparative politics, and local governance, as well as to researchers of authoritarianism, illiberalism, multilevel governance, and city diplomacy. It will also appeal to policymakers and practitioners concerned with the evolving role of cities and regions in an increasingly contested international system.