Valentia: Rome's Lost Fifth Province in Britain

Historians debate the location of Valentia, a lost fifth province of Roman Britain. Three primary sources mention it: Ammianus Marcellinus's account of Theodosius the Elder's campaign in AD 367, the Notitia Dignitatum listing five provinces around AD 400, and Polemius Silvius's reference to 'Valentiniana.' The province was likely created after Theodosius recovered territory from barbarians. The four known provinces—Britannia Prima, Britannia Secunda, Flavia Caesariensis, and Maxima Caesariensis—covered all land south of Hadrian's Wall, leaving no room for Valentia within that area. A leading theory places Valentia north of Hadrian's Wall, between it and the Antonine Wall, a region previously under Roman control in the 2nd century AD. This interpretation fits Ammianus's description of recovering lost territory and the Notitia Dignitatum's separate listing. The possible capital may have been Newstead (Trimontium). No definitive evidence exists, and the mystery persists.