Why do transport investments induce polycentric industrial growth in specific instances, rather than reinforcing existing cores? By investigating large-scale Greater Mekong Subregion highway projects in Northern Vietnam, where postwar state-led reconstruction restored monocentric urban structure following wartime devastation, this paper shows that later corridor investments generated nonlinear, polycentric industrial growth. Improved market access induced large manufacturing entry in agrarian peripheries, with effects amplified by local supply-chain depth. A spatial general equilibrium model with input-output linkages replicates this mechanism. The corridor raised income but widened spatial inequality, a trade-off mitigated by feeder-road complementarity. Infrastructure acts as a spatial big push only when connectivity and production networks jointly trigger new clusters. Overall, a structural shift from monocentric battlefield resilience to polycentric market expansion is catalyzed by infrastructure investments.