The spark that ignited into war between the United States and Mexico resulted from the dispute of where the boundary lay between the United States and Mexico. President James Polk ordered troops to the Rio Grande. In April 1846, Mexican troops crossed the Rio Grande and attacked a detachment of United States cavalry. On May 13, 1846, Congress declared war on Mexico. Results of Mexican-American War Over 13,000 Americans killed in battle or dead from disease. Northern Mexico and Texas added to the United States (virtually half of Mexico's territory). From Tripoli to the Halls of the Montezumas During the assault on Chapultepec and Mexico City, many Marines performed heroic deeds. When General Quitman's advance was halted by heavy enemy fire, Marine Captain George H. Terrett, whose Company C formed the right flank of the support, moved forward without orders and sliced through enemy batteries, pursued the fleeing artillerymen, and broke up a counterattack by Mexican lancers. With the fall of the stronghold at Chapultepec and one more day of fierce fighting by Marines and soldiers, Mexico City was taken by General Scott's forces on September 14, 1847. As a result of the Marines' participation in the Mexican-American War, the citizens of Washington, D.C., presented Commandant Henderson with a blue and gold standard which bore the motto, "From Tripoli to the Halls of the Montezumas."   Crucible for Greatness "The Mexican-American War stands for many things, admirable and not so admirable, in American history. But easily one of its most important and durable legacies was as the military laboratory and workshop, proving ground, crucible, and rite of passage for the men who would command on both sides in the American Civil War." "Those who absorbed the lessons offered by the Mexican-American War and then later used them, went on to greatness in the Civil War. A handful of them -- Lee, Grant, Jackson -- stand today in the upper pantheon of great American generals. They stand there in large part because of what they learned in Mexico on the National Road to the Halls of the Montezumas."   —John C. Waugh The Mexican-American War: Crucible for Greatness

Battle of Palo Alto, by Carl Nebel, 1850. Fragment of the lithograph printed by Lemercier, in París, for George W. Kendall's, The War Between the United States and Mexico (Illustrated), Nueva York, 1851. Westward the Course of Empire Taken Its Way, by Emmanuel Leutze, 1861. Fragment of the canvas that belongs to the National Collection of Fine Arts of the Smithsonian Institution.