The city of Rijeka/Fiume underwent an array of transitions in the long twentieth century, from the port of Hungary in the Dual Monarchy to a free city, to D´Annunzio´s Italian Regency of Carnaro, annexation by Italy, incorporation into Yugoslavia, and eventually the independence of Croatia. The article examines the processes of urban reconstruction and architectural reconfigurations in the city as “frontier urbanism”, building on Wendy Pullan’s (2011) discussion of how various actors employ architectural and place-making practices to secure the state in contested urban space. The article traces Rijeka/Fiume´s urban development as a window of fixating state identities in the built environment throughout the century, focusing on the aftermath of the Second World War. It examines the urban transformations of the city as the demographic landscape was reshaped after the departure of the local Italian-speaking majority and the arrival of workers from various parts of Yugoslavia, but also from Italy. By analysing decisions to rebuild or not buildings damaged by war, as well as the demolition of the 1943-built votive temple in Mlaka, the article inquires how reconstruction and urban planning became avenues to secure the state at its new frontiers.