From tax state to protection market: fortifications, coercion radius, and the spatial reorganization of power in post-Roman Europe, c. 400–900

The end of Roman rule in the West reorganized the spatial foundations of political economy. The late Roman Empire sustained power through a tax state that joined land assessment, coercive enforcement, military provisioning, judicial authority, and long-distance logistics within a relatively continuous geography of rule. Western contraction did not erase Roman forms: law, cities, episcopal authority, offices, titles, fiscal memory, and imperial prestige remained usable. They no longer worked at the same distance. Between the fifth and ninth centuries, authority had to be made credible from defended places: fortified cities, hillforts, monasteries, bridgeheads, castra, estate complexes, and burhs. We describe the surrounding zone of effective command as a coercion radius: the area within which a fortified node could enforce judgment, compel labor, tax movement, organize defense, and threaten force. Within those radii, rulers, bishops, monasteries, urban elites, and landed magnates exchanged protection for rents, tolls, service, labor, repair duties, transport, and obedience. That exchange formed a protection market whose structure linked military geography to emerging territorial rule. Drawing on late Roman law, Cassiodorus, Procopius, Gregory of Tours, Visigothic law and councils, Frankish capitularies, the Burghal Hidage, and scholarship on continuity, rupture, and regional transformation, we recast the passage from empire to successor polities as a reorganization of reach. Fortifications did not cause a new order by themselves. They made inherited and emerging forms of command defensible, repeatable, and locally credible after the Roman tax state lost western depth.