The Mexican-American War was a conflict between 1846 and 1848 sparked by territorial disputes following the US annexation of Texas. The war—America’s first fought mainly on foreign soil—involved around 150,000 troops fighting across present-day Texas, California, New Mexico, and deep into central Mexico.
Hours of research by our editors, distilled into minutes of clarity.
In just two years, the United States redrew the map of North America. Sparked by border disputes in Texas and fueled by President James Polk’s drive for expansion, the Mexican–American War (1846–1848) reshaped both nations.
Mexico’s post-independence instability—marked by coups, power struggles, and weak central control—left its northern frontier vulnerable. That political fragmentation shaped Mexico’s unpreparedness during the Mexican-American War and helped enable US conquest.
Mexico abolished slavery in 1829, but many Anglo-American settlers in Texas brought enslaved people and refused to comply. Their desire to preserve slavery was a major factor in Texas’s 1836 rebellion and independence from Mexico.
After Texas won independence from Mexico in 1836, the US annexed it in 1845—despite Mexico’s refusal to recognize Texas as separate. That annexation, combined with a heated border dispute, triggered the Mexican-American War the following year.
The belief that the United States had a divine right to expand westward—known as manifest destiny—justified President Polk’s push for territorial conquest. It shaped public support for the Mexican-American War and framed the invasion as national destiny.
After Mexico refused to sell California and New Mexico, President James K. Polk ordered US troops into disputed territory between the Nueces River and the Rio Grande—knowing it would likely spark a conflict. When fighting broke out, he used it to justify declaring war.
As a freshman congressman, Abraham Lincoln introduced the “Spot Resolutions,” demanding President Polk prove that Mexico had attacked on US soil. This constituted Lincoln’s first major political stand and highlighted growing dissent over the war.
Henry David Thoreau refused to pay taxes that funded what he saw as an unjust war waged to expand slavery. His act of protest inspired his famous essay Civil Disobedience, a foundational text for nonviolent resistance.
Despite being outnumbered in key engagements, US forces defeated Mexican armies across every major front. Superior training, leadership, and logistics allowed the US to win decisively—capturing Mexico City without losing a single major battle.
American volunteer regiments frequently looted towns, raped civilians, and executed prisoners—especially in occupied northern Mexico. Historians have documented how racial hatred and impunity fueled war crimes that were rarely punished by military leadership.
Weave together the many narratives of world history with our highly curated and expanding selection of diverse, fascinating resources designed to showcase the breadth and richness of Earth's story, from the earliest traces of human civilization to the dramatic developments of contemporary cultures.