I've recently been considering the Pax Americana in general, and the first World War in particular, in my spare time. Comparisons have been drawn in these forums between the years leading to that conflict and our own time, and I believe I have an original thought: Every great nation's existence will be challenged by another nation that wants to be a great nation. Ours has been an exception to this rule.

Cold War? No. I don't believe our opponents were prepared to simply blank themselves out of existence in a radioactive, mutually-destructive blink of an eye. Besides, some of America's most prosperous economic years are marked in this period for a great many people.

American Revolution? No. The people that advocated for and fought a bloody civil war (that's ...