Are you saying the south didn't want to preserve slavery? You'll note your definition of imperialism doesn't include anything about right or wrong, just one nation imposing it's will on another. This is what people mean when they describe liberal politics as vibe based - it fits the definition exactly, but you don't feel it's imperialist because it was the morally right thing to do.

While America wasn't the imperialist power in the world wars, they were thoroughly imperialist wars, by either definition, between European countries.