Carroll and James F, Tautologies shed no light on anything except perhaps the style of argument of those who use them. You're saying that radicals by definition are anti-imperialist, to which there's no reply, except to recall that many self-defined radicals, yesterday and today, see America as a God-given blessing not terrible ugly American (apologies to my favorite story-teller, Graham Greene). The tautology obscures the basic question: How is it that some people some of the time can take very radical stances, in fact, risk their lives and honor, for a domestic cause, still believing that America is not imperialist, but just makes mistakes, etc.
I add that I don't find the expression US imperialism used very much at all by lbo-ists. Because it's so obvious? Maybe. But I find rarely any discussion of the nature of, contradictions of, evils of, etc., US imperialism. So many Populists were imperialists, it would curl your hair, and that's a major radical movement. Check out William A. Williams work. And there were self-defined cold war radicals as well as cold war liberals. If radical means going to the roots of things, clearly the roots are so twisted, mixed-up with one another, numerous, etc., that one can have arguable evaluations of the nature of the US in the world.
Jim O'Connor