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Valentinian following the lead of Diocletian

Diocletian's choice of a new Roman capital at Nicomedia ("Victory of Media", i.e., the East) reflected the shift in power from West to East. Diocletian had evidently used his strength in Media-Persian to re-unite the Empire. Valentinian was simply re-asserting the seniority of the East established by his probable father or grandfather, Diocletian. The north-south shifts of power in Old Testament times were replaced by east-west shifts in the Christian Era.

http://en.wikipedia.org/wiki/Nicomedia