To be fair, the Mongol empire did end up disintegrating in a few generations. Kublai Khan was the de jure ruler of a united empire, but in practice the western periphery of the empire was drifting away.

> If you want to keep the empire unified, you need a civilian government and civilian institutions to keep the military under control.

But then you run into another classic political science problem: how do civilian institutions keep the military under control? Why can't the military simply shoot the civilians and institute a junta?