The lesson of history, as our president might say, is that all things must come to an end, including nations and empires. The empire falls first and then, sometimes, the nation itself. The American empire has been in decline since — oh, roughly, the end of the Vietnam War or thereabouts. The country’s power, prestige and sense of self waned badly as the final helicopters left the rooftops in Saigon. Except for a brief period during the Reagan administration, it’s been pretty much downhill ever since. …