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Expansion (1820-1849) During the early 1800's, settlers moved westward over the Appalachian Mountains into the new states and territories. ... Americans also settled in the Oregon Country, a large territory between California and Alaska claimed by both Britain and the United States.
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Westward expansion began in earnest in 1803. Thomas Jefferson negotiated a treaty with France in which the United States paid France $15 million for the Louisiana Territory – 828,000 square miles of land west of the Mississippi River – effectively doubling the size of the young nation.