In 1848, the year Zachary Taylor (1784-1850; see entry in volume 2) was elected president, the Treaty of Guadalupe Hidalgo officially ended the Mexican War (1846-48). In addition to settling the border of Texas, which was annexed to the United States in 1844 (see John Tyler primary source entry in volume 2), the treaty gave the United States additional territory in the Southwest (areas of present-day New Mexico, Arizona, and California ). During that period of the administration of President James K. Polk (1795-1849; see entry in volume 2), the United States also won possession of the Oregon Territory that forms the present-day states of Oregon, Washington, and Idaho.

The large increase in territory meant that new states would soon be entering the Union. That prospect further enflamed debate over the nation's most divisive issue—slavery. Since the Missouri Compromise of 1820 (see box...