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A war fought between the United States and Mexico from 1846 to 1848. The United States won the war, encouraged by the feelings of many Americans that the country was accomplishing its manifest destiny of expansion. Mexico renounced all claims to Texas north of the Rio Grande and yielded a vast territory that embraces the present states of California, Nevada, and Utah, and parts of Arizona, Colorado, New Mexico, and Wyoming.
Note: Many generals of the Civil War, including Ulysses S. Grant and Robert E. Lee, gained experience in battle during the Mexican War. The Mexican War was opposed by many Americans, notably by the author Henry David Thoreau, who was put in jail for refusing to pay a tax to support the war. His essay “Civil Disobedience” explains the principles of his action.