Let us play a game, shall we?
What declining historical unit are we?
Are we the Roman Republic, falling prey to greater and greater dictators, frittering away our power for bread and circuses?
Are we the Roman Empire, economically failing and geographically rent apart?
Are we Byzantium, ultimately facing attacks from both our enemies and our supposed friends?
Are the Latin States of the East, a bridge too far, unwilling to play the games we need to survive yet always insisting we come back to war?
Are we the Austro-Hungarian Empire, finally rent apart by nationalisms too fierce to be subsumed in the larger unit as they were before?
Are we another one - something you know and I do not (as you will note, my history is largely Western European centric)?
You note that I assume that we are, in fact, a declining historical unit. A quick read on any of these will, interestingly enough, point not only to the waning power of the unit but usually to the unfortunate financial and economic circumstances they found themselves in. Economic instability, it seems, ultimately ensures a state cannot weather the tides of its time.
My guess? Call me old fashioned, but I will go with the Roman Republic. We seem to more and more find ourselves in the position of having less and less power to influence our society, swept aside (ultimately) by the dictates of fewer and fewer individuals (in my mind, both major parties have abandoned us to this fate. It is not one or the other - nor was it in the Republic. If it would not have been Caesar, it would have been Crassus or Pompey, had they survived).
Rome, of course, ultimately survived. The Roman Republic, of course, did not - except in name only.