One of the most prominent philosophy of the American History was the Manifest Destiny, which the popular belief of the American that it is their god-given destiny to conquer the American continent, from coast to coast, "from sea to shining sea." The idea was started by John O'Sullivan, who wrote an article about it. James Polk is considered the Manifest Destiny President as he was the President that allowed California to be a state.

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Manifest Destiny potrait
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John O'Sullivan
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James Knox Polk

Article:
http://www.mtholyoke.edu/acad/intrel/osulliva.htm