Brad Plumer's got a characteristically thoughtful post on why Americans want to be the dominant global power. I mean, really, what good does it do us? And, from a logical standpoint, he's right. In fact, I'd much rather be a highly-developed country on the second-tier of world power (like Japan or France) than America. So long as you believe the global strongman to be basically benevolent -- and, odd bouts of French-hating and Japanophobia aside, we've proved ourselves such -- you're really in much better shape letting someone else worry about supporting a massive army, purchasing all the latest weapons technology, and dashing across the globe when the bat eagle signal dances across the sky. You can save your cash and create a nice, comfy social net, full of health care for all and long, paid vacations.
But, if you're an American, and you've got even an ounce of nationalism in you, you'd rather see a world dominated by you than anyone else. You are good, you don't believe there to be anyone better, so the best possible outcome for the world, if not for your deficit, is American preeminence. in that way, I think Brad underestimates how much of this is a weird offshoot of white man's burden, call it best country's chore. It's not that Americans really want to be dominant, it's that they feel they kinda owe it to the world, all things considered.
Aside from some folks at the top of the national security food chain who defend it in cost-benefit terms, I think this is mostly an instinctive thing. Americans are a bit isolationist and a bit reluctant to project force in order to promote Good Stuff, but they'll generally vote for the hegemonically-inclined over his opponent, because this taps into nationalism rather than foreign policy opinion. And that's why I believe any effective national security critique the Democrats can adopt will rely, in large part, on arguing that there are better ways to promote American preeminence and apply American force. I don't think you'll get far by tapping into isolationist strains, but I think you can make quite a run by invoking our moral authority and responsibility to set an example for the world.
(If there's one thing we know about comment trolls, it's that they're lazy)