The Mexican-American War (1846-1848) played a critical role in shaping U.S. territorial expansion. Rooted in the U.S. desire to acquire Texas and further westward, the conflict resulted in significant territorial gains for the United States through the Treaty of Guadalupe Hidalgo. This agreement ceded vast territories, including present-day California, Arizona, New Mexico, and Texas, effectively fulfilling the concept of Manifest Destiny. However, the war exacerbated tensions over slavery, contributing to sectional divisions that would ultimately lead to the Civil War and setting the stage for future legal and political conflicts regarding territorial governance.