You seem to be under the impression that empire and imperialism means annexation.
That is not the case. The exploitation and oppression abroad is a huge factor. The US alone, for example, are imperialist with or without NATO, as they influence, oppress, exploit and even kill people all over the world for the sake of profits. They never annex these countries though.
NATO is relevant, because it is an extension of US imperialism and a big alliance of other imperialist countries, spreading this exploitation and oppression with the help of fascists even.
Unreal and Unreal Tournament
Saved you a click.