What means and methods (military, technological-industrial, and political-diplomatic) supported by what reasons and ideas (economic, racial, ideological, and religious) did the United States gain so much territory and from whom?

The United States expanded greatly from 1803 to 1853 through a variety of means and methods supported by important reasons and ideas. By what means and methods (military, technological-industrial, and political-diplomatic) supported by what reasons and ideas (economic, racial, ideological, and religious) did the United States gain so much territory and from whom? Who (individuals and groups) went west and why?
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