Western Expansion, Manifest Destiny, and the Mexican-American War

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What was the genesis or cause of the "westward expansion" of the United States?

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The origins of westward expansion by the newly-established United States actually predate the nation’s history and represent the natural tendency of mankind to explore and conquer uncharted territories.  British, French and Spanish explorers landed in and subsequently settled new territories in the Americas and the expansion beyond initial settlements was, for all intents and purposes, a natural human manifestation – and provided the genesis of “Manifest Destiny,” the name derived from newspaper editor John O’Sullivan’s 1845 declaration that it was America’s “. . . right of manifest destiny to over spread and possess the whole of the continent which Providence has given us for the development of the great experiment of liberty and federative development of self-government given to us.”

Well-before O’Sullivan’s declaration of divine intervention for the United States, however, westward expansion was already the natural inclination of the country’s first leaders.  President Jefferson’s 1803 purchase of a vast expanse of territory from France – the Louisiana Purchase – was the first significant manifestation of westward expansion, but it was, of course, not the last.  As Jefferson noted,

“I know the acquisition of Louisiana has been disapproved by some . . . that the enlargement of our territory would endanger its union . . . The larger our association the less will it be shaken by local passions; and in any view is it not better that the opposite bank of the Mississippi should be settled by our own brethren and children than by strangers of another family.”

While the natural expansion of the United States could be considered pre-ordained by virtue of the innate tendency of people to explore and settle, there was also a significant geopolitical component to the policy of westward expansion.  While relations with France had been good by virtue of the mutual interest in containing or rolling back British possessions, France and Spain both represented potential rivals and threats to the still emerging country now led by Jefferson.  Jefferson’s dispatch of the Meriwether Lewis and William Clark-led expedition was, in essence, the deployment of a long-range reconnaissance team for the purpose of scouting possible threats and opportunities well-beyond visible range.  The later expression of westward expansion as a matter of “manifest destiny” was simply the articulation of policies and practices already well-established.

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