March 10
U.S. Senate Ratifies Treaty Ending Mexican-American War
The Senate's 38–14 vote on March 10, 1848, approved the Treaty of Guadalupe Hidalgo, ending the Mexican-American War and transferring more than half a million square miles of Mexican territory to the United States.
Summary
The Mexican-American War, sparked by disputes over Texas annexation and border claims, had lasted nearly two years when negotiator Nicholas Trist secured the Treaty of Guadalupe Hidalgo in February 1848 despite recall orders from President Polk. The agreement called for Mexico to cede vast territories including present-day California, Nevada, Utah, and parts of several other states in exchange for $15 million and debt relief. On March 10, 1848, the U.S. Senate ratified the treaty by a vote of 38 to 14 after contentious debate over territorial extent and slavery implications. Ratification cleared the path for formal exchange of instruments and proclamation later that year. The treaty redrew the map of North America and intensified debates over expansion and slavery.
Context
Tensions between the United States and Mexico had simmered since Texas declared independence in 1836 and then sought annexation. President James K. Polk, elected on an expansionist platform, completed Texas statehood in December 1845 and ordered troops under Zachary Taylor into the disputed area between the Nueces and Rio Grande rivers. When Mexico rejected U.S. efforts to purchase California and New Mexico through envoy John Slidell, Polk used border skirmishes to secure a declaration of war in May 1846.
What Happened
U.S. forces under General Winfield Scott captured Mexico City in September 1847, forcing the Mexican government to negotiate. Polk’s emissary, State Department chief clerk Nicholas Trist, stayed in Mexico despite a recall order issued in October and reached agreement with Mexican commissioners Luis Gonzaga Cuevas, Bernardo Couto, and Miguel Atristain. The resulting Treaty of Guadalupe Hidalgo, signed February 2, 1848, at the village of Guadalupe Hidalgo, required Mexico to cede roughly 525,000 square miles—including present-day California, Nevada, Utah, New Mexico, most of Arizona and Colorado, and parts of other states—in exchange for $15 million and U.S. assumption of up to $3.25 million in Mexican debts to American citizens; it also fixed the Texas boundary at the Rio Grande.
Aftermath
Polk submitted the treaty to the Senate, where debate centered on the extent of the cession and the future of slavery in the new lands. On March 10 the Senate ratified the document 38–14, striking Article X that had protected Mexican land grants and adding other amendments. Ratifications were exchanged at Querétaro on May 30, U.S. troops withdrew, and President Polk proclaimed the treaty on July 4, 1848.
Legacy
The acquisition completed the continental expansion envisioned by Manifest Destiny and established the modern U.S.–Mexico border framework later adjusted by the Gadsden Purchase. It also intensified sectional conflict over slavery’s extension, helping set the stage for the Civil War while creating enduring questions about land titles, citizenship, and indigenous rights in the ceded territories that continue to shape legal and historical discourse.
Why It Matters
Ratification formalized the largest territorial acquisition in U.S. history up to that point, fulfilling much of the vision of Manifest Destiny while fueling sectional tensions that contributed to the Civil War. It established enduring border frameworks and legal precedents still referenced in land and immigration discussions today.
Related Questions
Why did Nicholas Trist ignore his recall order?
Trist believed he had a final opportunity to secure peace and feared that further delay would prolong the costly war and risk losing the chance for a treaty.
What territory did Mexico cede under the treaty?
Mexico relinquished more than 525,000 square miles, including the present-day states of California, Nevada, Utah, New Mexico, most of Arizona and Colorado, and portions of Oklahoma, Kansas, and Wyoming.
How did the treaty affect the debate over slavery?
The vast new territories reignited arguments over whether slavery would expand westward, contributing to the sectional crisis that led to the Civil War.
What was the financial settlement in the treaty?
The United States paid Mexico $15 million and assumed up to $3.25 million in debts owed by Mexico to American citizens.
Did the Senate make changes to the original treaty?
Yes, the Senate struck Article X protecting certain Mexican land grants and added other amendments before ratification.
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Sources
- Treaty of Guadalupe Hidalgo, U.S. National Archives. Accessed 2026-07-08.
- The Annexation of Texas, the Mexican-American War, and the Treaty of Guadalupe-Hidalgo, U.S. Department of State, Office of the Historian. Accessed 2026-07-08.