The Mexican–American War, was an armed conflict between the United States of America and Mexico from spring 1846 to fall 1847, resulting in the defeat of Mexico and the loss of approximately half of its national territory in the north.
After its independence in 1821 and brief experiment with monarchy, Mexico became a republic in 1824. It was characterized by considerable instability, leaving it ill-prepared for conflict when war broke out in 1846. Native American raids in Mexico’s sparsely settled north in the decades preceding the war prompted the Mexican government to sponsor migration from the U.S. to the Mexican province of Texas to create a buffer. However, Texans from both countries revolted against the Mexican government in the 1836 Texas Revolution, creating a republic not recognized by Mexico. In spite of its de facto secession in the revolution, Mexico still claimed it as part of its national territory. In 1845, Texas agreed to an offer of annexation by the U.S. Congress, and became the 28th state on December 29th.
In 1845, James K. Polk, the newly-elected U.S. president, made a proposition to the Mexican government to purchase the disputed lands between the Nueces River and the Rio Grande. When that offer was rejected, American forces commanded by Major General Zachary Taylor were moved into the disputed territory. They were then attacked by Mexican forces, who killed 12 U.S. soldiers and took 52 as prisoners. These same Mexican troops later laid siege to an American fort along the Rio Grande. This led to the war and the eventual loss of much of Mexico’s northern territory.
U.S. forces quickly occupied Santa Fe de Nuevo México and Alta California Territory, and then invaded parts of Central Mexico which modern-day Northeastern Mexico and Northwest Mexico. The Pacific Squadron conducted a blockade and took control of several garrisons on the Pacific coast farther south in the Baja California Territory. The U.S. Army, under the command of Major General Winfield Scott, captured the capital, Mexico City, marching from the port of Veracruz.
The 1848 Treaty of Guadalupe Hidalgo ended the war and specified its major consequence: the Mexican Cession of the territories of Alta California and Santa Fe de Nuevo México to the United States. The U.S. agreed to pay $15 million compensation for the physical damage of the war. In addition, the United States assumed $3.25 million of debt owed by the Mexican government to U.S. citizens. Mexico acknowledged the loss of Texas, the Pecos River as their border and thereafter cited the Rio Grande as its national border with the United States.
In Mexico, the war came in the middle of political turmoil which turned into chaos during the conflict. The military defeat and loss of territory was a disastrous blow, causing Mexico to enter “a period of self-examination” as its leaders sought to identify and address the reasons that had led to such turmoil and loss.” A prominent Mexican wrote the war had resulted in “the state of degradation and ruin” in Mexico.