You can make up any story about one of The Bad Countries, and as long as it's not blatantly in the realm of fiction (e.g., space werewolves secretly control Iran) a significant majority of Americans will at least think it's plausible. I don't know if this is more of a human thing than an American thing, but it's definitely a factor in the United States.

What this means is that the road to anti-imperialism (not just opposition to one particular act of imperialism) -- for most Americans -- will run through some form of leftism or not at all. You can get a non-leftist to nod along to a lot of anti-imperialism, but they'll readily backslide the moment a "China bad" or "Iran bad" story hits. The stumbling block is that non-leftists don't believe the vast bulk of the U.S. government is capable of being comprehensively, totally in the wrong on purpose. They're constantly looking for some excuse to say that we were at least partially justified in this particular act of imperialism, or they're looking for some internal U.S. boogeyman that can be removed to fix the problem. They see U.S. empire as a machine that can be fixed, not a machine designed for a terrible purpose that's working as intended.

The way to get past this stumbling block is to get them on board with leftist solutions to domestic problems; problems they actually know and care about, where their personal experience insulates them from buying into outlandish propaganda. Make them a leftist on domestic issues and it'll be far easier to make them an anti-imperialist on foreign issues.