Complex sentence: Although Augustus could claim that his power was Republican, by the time Tiberius inherited the throne this claim could no longer be convincingly made, and, by the time of Diocletian, any pretence that the Emperor was not a monarch had been shed.
Split into simpler sentences: Although Augustus could claim that his power was Republican, his successor, Tiberius, could not convincingly make the same claim. By the time of Diocletian, any pretence that the Emperor was not a monarch had been shed.