On the Fact that Nation-States Are Rarely Murdered and Almost Always Decay from Within (Through Subversion, a Debasement of its Currency, Decadence, Corruption, Factionalism, etc.)

And even in the case of Rome, yes, they were overrun by Germanic armies but Rome at its apex would have repelled these barbarians with relative ease. It wasn't until Rome debased its currency, experienced a disintegration of its economic interconnectedness, and became a moral basket-case that it became ripe for the picking (Odoacer retaining some of the better elements of Rome's polity adding a wee-bit of irony to the mix for sure). Thankfully there aren't any barbarians at America's gate just yet but if history is any indication......