>a polity ruling over huge territories and many people, in which rule is not based on explicit consent by the governed; it is a very great power that defines the international system and is linked to a great culture.
>In that sense, the charge could definitely be made that the US is - intentionally or not - an empire:

I think Canada and the US (and possibly Mexico) are empires by that defintion, but perhaps not in the way you meant. The US and Canada started as small countries on the east coast of North American then expanded via the use of military force, military strength and coercion.