In less than 50 years, the western border of United States grew from the Mississippi River to the Pacific Ocean, starting with the Louisiana Purchase and ending with the territories gained from Mexico in the Mexican-American War. Spain gave up its claim to Florida and the northeastern Gulf Coast, and agreements with Great Britain established the boundary with Canada by 1846.

Map by National Geographic Society

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  • On December 30, 1853, the United States and Mexico established their existing border with the completion of the Gadsden Purchase. Only five years after the United States acquired most of the present-day states of California, Arizona, Utah, and Nevada following the Mexican-American War, the U.S. completed the Gadsden Purchase for $10 million. The Gadsden Purchase provided the U.S. with what is now southern Arizona and southwestern New Mexico.

    Originally, the U.S. wanted to build a transcontinental railroad through the new territory, using slave labor. The debate over slavery was already so contentious that the project received no federal funding, and the first transcontinental railroad was ultimately built farther north.

  • Term Part of Speech Definition Encyclopedic Entry
    acquire Verb

    to get or take possession of.

    border Noun

    natural or artificial line separating two pieces of land.

    Encyclopedic Entry: border
    establish Verb

    to form or officially organize.

    transcontinental railroad Noun

    railroad that spans an entire continent.