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The possession of territories west of the Mississippi River stemmed from a U.S. impulse to expand the national domain, which became apparent during the Revolutionary War when Continental soldiers invaded Canada. The western states evolved in three phases: acquisition, territorial, statehood. By 1854 the boundaries of the continental United States had been fleshed out through a mix of political processes, including purchase, treaty, conquest, and legislation. The annexation of the noncontiguous states as territories in 1867 and 1898 completed the expansion of the public domain. At different times, these territories were variously claimed by indigenous tribes, France, Spain, Great Britain, Mexico, and Russia.
The United States began its march toward the Pacific Ocean in 1803 when President Thomas Jefferson accepted the offer from France to purchase ...