I think it's wrong to talk about "Spanish Imperialism" as "acting" in any way. It was the Spanish _government_ that supported Pinochet -- and at the time, the Spanish government was under the thumb of a kindred spirit, Generalissimo Francisco Franco.
It's crucial to remember that, as some old Russian guy pointed out, imperialism is not the same thing as government policy. It's a phase of capitalist development, i.e., one form that the social system of capitalism takes in its development. (I think this applies not only to the kind of turn-of-the-20th century imperialism that Lenin, Hobson, Luxembourg, and Bukharin described and analyzed but also to the current version and the version that prevailed in 1973.) Now, as a social system, it helps _generate_ certain government foreign policies.[*] But it is not the same thing.
And Spain and England are just small parts of the imperialist system. In fact, it's not strictly correct to talk about "US imperialism" since the US is only a part. However, since the US is still the imperialist hegemon, it seems acceptable shorthand to me.
[*] An example of generation of certain policies by the system: look how Pres. Clinton -- seemingly the most powerful person in the world, perhaps #2 after Greenspan -- has changed his spots as he has adapted to the pressures and pulls of the politics that the eternal welter of money investment by the various fractions of capital -- in combination with the concrete situation of the system. He went from "it's the economy stupid" (mild reformism) to neoliberal triumphalism to crisis management.
Jim Devine jdevine at popmail.lmu.edu & http://clawww.lmu.edu/Departments/ECON/jdevine.html