Les, much of the expansion of the United States during the 18th and 19th Centuries had to do with the concept of Manifest Destiny, the notion that the United States was to eventually encompass the entire North American land mass, replacing the European Colonial territories with free and democratic rule. This eventually failed in the case of Mexico and Canada because of two factors : American military inferiority to the British and the people's loyalty to the Crown in the case of Canada, and to American popular opposition and the physical barrier of desert wasteland in the case of Mexico. For better or worse, Native American populations were never perceived as "Nations" to be conquered. Territory inhabited by Indian tribes was claimed by countries like Britain, France, or Spain, who were seen as competitors to the emerging nation of the United States. These territories were annexed through war, compromise, and purchase.

In case you are of the opinion that the expansion of the US was a clear case of greed and land-grabbing, you need to keep one thing in mind: The United States was a new phenomenon in the 19th Century, a Republic whose leaders were elected by the citizens. This was a radical and threatening new concept to the European Monarchies, and though the French initially backed the revolt of the American Colonies, there was no European State who did not have some interest in seeing this democratic experiment fail. If the Russian Revolution was threatened by other imperial forces surrounding and meddling in its affairs, the US was at least as vulnerable when it viewed the British, Spanish, and French adjoining its borders. Expansion was, in many ways, a form of self-defense.