3. The Mexican-American War (1846-1848)
Spurred by a desire for expansion, the Mexican-American War led to the U.S. acquiring vast territories, including California and the Southwest. The Treaty of Guadalupe Hidalgo ended the conflict, establishing the Rio Grande as the southern boundary of Texas and ceding significant land to the United States. This acquisition fueled debates on slavery and significantly shaped U.S.-Mexico relations. The controversial legacy of Manifest Destiny was also reinforced during this period (britannica.com).
