I think you views are a little optimistic.
And your desire to lump the USA into the definition of an "Imerialist" country is clouding your view of just how a truely imperialist nationstate would act in a modern technological environment if that imperialist nationstate were not checked by some other nationstate that valued liberty over power.
Kick the USA and her motives if you must, but the Earth would be a nuclear fallout ravaged disaster zone were it not for the USA role as ultimate "peacekeeper".
Like it or not, the lawman sometimes needs to use deadly force to keep the peace: So the USA invaded Iraq to keep the peace.
If choose to believe otherwise, well--that's your problem.