Empires can suffer military defeats, economic collapses, even civil wars (they were pretty much a national sport for the Romans) but what kills them off for goo

Empires can suffer military defeats, economic collapses, even civil wars (they were pretty much a national sport for the Romans) but what kills them off for good are crises of legitimacy. When people no longer believe in the unifying ethos of the empire, it's game over. For example, when people stopped believing in Communism and the institutions of the Party, the USSR was done. All the nuclear weapons and KGB goons couldn't keep it together. When Roman citizens figured out that the legions would no longer protect them and they were better off paying tithes to a local feudal lord rather than taxes to Rome, the Roman Empire was done. And in the US, if nobody believes in the democratic system of government and the only way to keep their party in power is through insurrection or voter suppression, then the US as we know it will be over as well.