6. The Mexican-American War and Treaty of Guadalupe Hidalgo
The Mexican-American War (1846-1848) concluded with the Treaty of Guadalupe Hidalgo, which added approximately 525,000 square miles to the United States, including present-day California, Nevada, Utah, Arizona, and parts of Colorado and New Mexico. This acquisition reignited debates over the expansion of slavery into new territories, leading to significant political compromises and intensifying sectional tensions that contributed to the onset of the Civil War (history.com).
