Stephen Green, Vodka Pundit, on Tuesday's Republican debate thread had some thoughts on the new American Imperialism in the comments.
Let's not forget also that America's western foreign policy was explicitly imperialistic -- using our excess farmers, and their desire for cheap, new lands -- to push our western border to the Mississippi and beyond.
And Jefferson wanted to add Canada, Mexico, and Cuba to America's lands. *Jefferson*.
For those that want to delve deeper in the subject there is the wiki.