In a.d. 286 Diocletian sought to reverse the decline by splitting the administration of the empire in two he would govern the east, based in Nicodemia, while his friend Maximian ruled the west from Milan and later to split it further into four parts.
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Diocletian and Maximian ruled the eastern and western parts of the empire, respectively.
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Diocletian split the empire in half because it was doing so well.