Stale and Crumbling
Empires are expensive entities. The bigger the Empire, the more the faces of the poor are ground into the dust. But as anthropologist Clifford Geertz pointed out, human culture is dense, rich, and exceedingly multi-layered. The problem with all imperial ambitions and achievements is that they can never encompass the richness and complexity of human ingenuity, creativity, or reason.
The intrinsic imperial instinct is to restrict, control, suppress, outlaw, forbid, and subdue. But in every human heart lurks a nascent revolution against the arbitrary edict. In every human heart lies a self-respect, an amour-propre, which sees right through the empty vanity of the emperor and his imperial edits. The empire can only be maintained if it keeps control of everything--which is impossible. Therefore, the more empires grow, the more impossible they are to maintain. Generally, empires crumble and collapse from within, crushed under the millstone of their rules and ever burgeoning edicts and rules, on the one hand, and forever being undermined by the subtle complexities of subjects, on the other.
When dictators and emperors emerge, one option in dealing with them is to give them enough rope so they hang themselves. The very complexity of human beings ensures that, given enough rope, it will inevitably happen. Blame the wondrous complexity of the created human soul. Nations and empires which eschew freedom and seek, instead, to command and rule by fiat over subjects, will inevitably collapse from the centre. The very multi-layered complexity of human beings ensures it.
If one would like a modern example, take the latter-day Soviet Union, Mark II.