Diocletian
Nicomedia in northwestern Asia Minor (modern Izmit in Turkey), a base for defence against invasion from the Balkans and Persia's Sassanids was the capital of Diocletian, the eastern (and most senior) Augustus. In the final reorganisation by Constantine the Great, in 318, the domain, facing Sassanid Persia, became the pretorian prefecture Oriens 'the East', the core of later Byzantium.