By the time the Oregon question was settled, the United States had entered into all-out war with Mexico, driven by the spirit of Manifest Destiny and territorial expansion.
The Treaty of Guadalupe Hidalgo, which ended the Mexican-American War in 1848, added an additional 525,000 square miles of U.S. territory, including all or parts of what is now California, Arizona, Colorado, New Mexico, Nevada, Utah and Wyoming.
Despite the lofty idealism of Manifest Destiny, the rapid territorial expansion over the first half of the 19th century resulted not only in war with Mexico, but in the dislocation and brutal mistreatment of Native American, Hispanic and other non-European occupants of the territories now being occupied by the United States.
U.S. expansion also fueled the growing debate over slavery, by raising the pressing question of whether new states being admitted to the Union would allow slavery or not—a conflict that would eventually lead to the Civil Wa
