The Mexican–American War (1846–48) was the U.S. Army's first experience waging an extended conflict in a foreign land. This brief war is often overlooked by casual students of history since it occurred so close to the American Civil War and is overshadowed by the latter's sheer size and scope. Yet, the War was instrumental in shaping the geographical boundaries of the United States. At the conclusion of this conflict, the U.S. had added some one million square miles of territory, including what today are the states of Texas, Arizona, New Mexico, and California, as well as portions of Colorado, Wyoming, Utah, and Nevada. This newly acquired land also became a battleground between advocates for the expansion of slavery and those who fought to prevent its spread. These sectional and political differences ripped the fabric of the union of states and eventually contributed to the start of the American Civil War, just 13 years later. In addition, the Mexican-American War was a proving ground for a generation of U.S. Army leaders who, as junior officers in Mexico, learned the trade of war and latter applied those lessons to the Civil War.
The Mexican–American War lasted some 26 months from its first engagement through the withdrawal of American troops. Fighting took place over thousands of miles, from northern Mexico to Mexico City, and across New Mexico and California.