Carrying on a useless arms race that cost us about 10% of our gross national product for at least a generation and the useless spending is still going on on our part years after the Soviet Union went broke from it.
Vietnam and Korea come to mind. Though as demonstrations of national will and power they were impressive. It's one thing to roll over relatively impotent nations like Iraq. It's another to take serious casualties and keep on coming. And there is nothing like killing a few million people to demonstrate that we can be murderous as all when the spirit takes us. But that isn't the kind of country we should want to be.