The Treaty of Guadalupe Hidalgo concluded the hostilities of the Mexican–American War, which had erupted in 1846 over territorial disputes following the annexation of Texas by the United States. Negotiated by U.S. envoy Nicholas Trist, the treaty was signed in February 1848 in the Mexican capital. The agreement represented a massive territorial gain for the United States, transferring approximately 525,000 square miles of land. This acquisition would not only alter the map of North America but also exacerbate the contentious issues surrounding slavery in the new territories.

The treaty faced significant opposition in the U.S. Senate, primarily from those concerned about the extension of slavery into the new territories. Notable figures such as Sen. John C. Calhoun opposed the ratification, fearing it would tip the balance of power in favor of free states. Ultimately, the Senate voted to ratify the treaty on March 10, 1848, with a final tally of 38 in favor and 14 opposed, ensuring its acceptance and paving the way for American westward expansion.

The ratification of the Treaty of Guadalupe Hidalgo marked a significant shift in U.S. foreign policy and territorial ambitions, leading to facilitated migration and settlement in the West. The legacy of the treaty is complex, as it left unresolved issues between the U.S. and Mexico and sowed the seeds for future conflicts over civil rights and land disputes in the newly acquired territories.