To explore one aspect of this, human nature is such that oftentimes one's enemies will attempt to do to oneself what was previously done to them. For example, the Assyrian tactic of targeted terror might have worked for a while to secure their empire: it could be that if one defeated one's enemies in the most horrific and brutal way possible, and then subsequently put down any further rebellions with the same heavy-handedness, then it was possible that this had a dampening effect on further resistance, actually lowering certain forms of violence and truncating possible wars.
This shorter term and more localized solution might have presented ostensibly promising results at a time when such a tactic hadn't yet been attempted over larger scales and time frames, but then over longer periods of time and at greater scale, there was that lingering problem that these transgressions would be stored in the memory of the oppressed, and as soon as there was a sign of weakness, say in the form of a faltering empire, then the conquered would return the favor in kind as soon as the opportunity presented itself. And so the Assyrian empire seemed to have unraveled in a reverse direction of its rise through retributive violence and disintegration: the empire seemed constantly ready to jump out of its skin, with building pressures of rebellion spreading throughout.
To use a simpler and more general metaphor, anyone who has set a mouse trap knows that the same amount of strength it took to set it would come back down in kind as it was released. The problem arises when, due to a mix of overreliance and growing senility, there are now so many traps set about and one begins to forget where they were or even what they are and how they work. The trap becomes another inherent hazard, as opposed to an asset.
The same general problem could be applied to any empire in a general sense, whether its tactics are softer or more heavy-handed, because an empire is typified by an overriding interest in the centralization of control and the related uneven distribution of resource, and the progressively more intense subjugation of those people further outside its circles of affinity and familiarity.
There are plenty of tendencies of following in human societies, with many all too willing to demure to what is perceived as a greater power that will serve their interests. But this relationship gets harder to sell when that greater power is at a greater remove, whether it be spatially or culturally or historically or what have you. Doubly more difficult the more obvious the disdain there is for diverse interests not shared by that power.
This is less meant to be a judgment than an observation. It is less apparent that a society's character is consciously and deliberately arrived at, and whatever is conscious and intentional is more often arrived at after the fact than not, though it is not an insignificant force. Further the necessities imposed by long-running historical forces external to any given society can be tremendous indeed.
To continue on with the proud tradition of muddying the waters, I'll come back to this soon with some complicating examples, as this pattern is less simple and more difficult to discern for those living the history.
