The Mexican War (1846-1848) 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 Mexican 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 thir#65533;teen years later. In addition, the Mexican 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 later applied those lessons to the Civil War.The Mexican War lasted some twenty-six 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. During the conflict, the U.S. Army won a series of decisive conventional battles, all of which highlighted the value of U.S. Military Academy graduates who time and again paved the way for American victories. The Mexican War still has much to teach us about projecting force, conducting operations in hostile territory with a small force that is dwarfed by the local population, urban combat, the difficulties of occupation, and the courage and perseverance of individual soldiers. The following essay is one of eight planned in this series to provide an accessible and readable account of the U.S. Army's role and achievements in the conflict.