We review recent evidence regarding decentralization and state strength and argue that decentralization can deepen democracy without compromising state strength if adequately designed. We examine how decentralization affects five key aspects of state strength: 1) Authority over territory and people, 2) Conflict prevention, 3) Policy autonomy and the ability to uphold the law, 4) Responsive, accountable service provision, and 5) Social learning. We provide specific reform paths that should lead to strengthening in each. Decentralizing below the level of social cleavages should drain secessionist pressure by peeling away moderate citizens from radical leaders. The regional specificity of elite interests is key. If regional elites have more to lose than gain from national schism, they will not invest in politicians and conflicts that promote secession. Strong accountability mechanisms and national safeguards of minority rights can align local leaders’ incentives with citizens’, so promoting power-sharing and discouraging local capture or oppression. “Fragmentation of authority” is a mistaken inference; what decentralization really does is transform politics from top-down to bottom-up, embracing many localities and their concerns. The state moves from a simpler, brittler command structure to one based on overlapping authority and complex complementarity, where government is more robust to failure in any of its parts. Well-designed reform, focusing on services with low economies of scale, with devolved taxation and bail-outs prohibited, should increase public accountability. Lastly, we advance a novel way that decentralization can strengthen democracy: by allowing citizens to become political actors in their own right, the small scale of local politics should promote social learning-by-doing, so strengthening political legitimacy, state-building, and ‘democratic suppleness’ from the grassroots upwards.