Mexican-American War Facts

Key facts about the Mexican-American War.

James K Polk, 11th President, Portrait

On May 11, 1846, President James K. Polk sent a message to Congress, proclaiming that “Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon American soil.” Two days later, Congress approved a declaration of war against Mexico. Mexican officials reciprocated by declaring war against the United States on July 7. Image Source: National Portrait Gallery.

Also Known As

  • The Mexican-American War is also known as the U.S.-Mexican War and the Mexican War.

Date and Location

  • The Mexican-American War lasted from April 25, 1846, until February 2, 1848.
  • The Mexican-American War was fought on four fronts, Northern Mexico, New Mexico, California, and Central Mexico.

Causes of the War

  • A primary cause of the Mexican-American War was a border dispute between the United States and Mexico along the Rio Grande and Nueces rivers in Texas.
  • A primary cause of the Mexican-American War was U.S. President James K. Polk’s firm belief that the United States was destined to expand its borders to the Pacific Ocean. Expansionist journalists adopted his stance and popularized the concept as America’s “Manifest Destiny.”

Primary Military Commanders for the United States

  • Primary American military commanders during the Mexican-American War were General Zachary Taylor, General Winfield Scott, Lieutenant Colonel John C. Frémont, Colonel Stephen W. Kearny, Commodore John D. Sloat, Commodore Robert F. Stockton, and Brigadier General John E. Wool.

Primary Military Commanders for Mexico

  • Primary Mexican military commanders during the Mexican-American War were Antonio López de Santa Anna, General Mariano Arista, General Pedro de Ampudia, Captain José María Flores, and General Nicolás Bravo.

Significant Battles of the War

American Armies That Participated

  • American armies that fought in the Mexican-American War were the Army of Occupation, Army of the Center, Army of the West, and the Army of Invasion.


  • United States casualties during the Mexican-American War totaled about 17,000 (13,000 dead and 4,000 wounded). Of the deaths, roughly 1,800 military personnel died in combat. The others died from sickness and disease.
  • Mexican casualties during the Mexican-American War totaled about 25,000, including possibly 1,000 civilians.

Treaty of Guadalupe Hidalgo

  • On February 2, 1848 representatives from the United States and Mexico signed the Treaty of Guadalupe Hidalgo, which concluded the Mexican-American War.
  • The Treaty of Guadalupe Hidalgo established the Rio Grande River as the southernmost border between Mexico and the United States. It also required Mexico to cede California, Arizona, New Mexico, and portions of Utah, Nevada, and Colorado to the United States, in return for a payment of $15 million.
  • The U.S. Senate ratified the Treaty of Guadalupe Hidalgo on March 10, 1848.
  • The Mexican Congress ratified the Treaty of Guadalupe Hidalgo on May 25, 1848.


  • Zachery Taylor’s military successes in Northern Mexico during the Mexican-American War buoyed his political ambitions, enhanced his popularity, and propelled him to victory in the 1848 U.S. presidential election.
  • On the home front in the United States, the Mexican-American War crystallized and polarized sectional differences over the extension of slavery in the United States.
  • The Mexican-American War was a training ground for numerous general officers who fought on both sides during the American Civil War.