The United States Marines raised the American flag over Mexico City -- the Halls of Montezuma -- on September 14, 1847, thus ending one of the nation's most successful and controversial wars.

Even today historians cannot agree on the reason why the Mexican-American War started. Some say Mexico started it because the U.S. annexed Texas in 1845, which Mexico still claimed despite the establishment of the Texas Republic ten years before. Others, however, say that America began the war, by stationing troops along the Rio Grande border between the two countries. Others say that the war was a plot by President James K. Polk to seize California.

Whatever the truth, on April 25, 1846, Mexican troops crossed the Rio Grande and attacked troops under General Zachary Taylor. Congress declared war in May, and Taylor invaded northern Mexico, winning victories over forces led by dictator General Antonio Lopez de Santa Anna.

Meanwhile, other troops under Colonel Stephen Kearny took Santa Fe and New Mexico. Then they marched into California, where American settlers under Captain John C. Fremont, had already declared their independence and set up the "Bear Flag Republic."

Finally, another American force landed at Veracruz on the Gulf of Mexico, took that city, advanced inland, and in a daring campaign -- completely cut off from supply or reinforcements -- took Mexico City.

On February 2, 1848, a peace treaty was signed at Guadalupe Hidalgo. Mexico handed over to the U.S. almost all of California, Arizona, New Mexico, and Utah. They also recognized the Rio Grande as the border between Texas and Mexico. The U.S. paid Mexico $15 million for the land and took over claims of American citizens against Mexico for property seized by that government.

Many Americans had opposed the war, including former President John Quincy Adams and future President Abraham Lincoln, who were both Congressmen, and a young Army officer named Ulysses Simpson Grant. Grant thought the war unjust, but he stayed in uniform and fought bravely. However, most Americans supported the war, which was fought entirely by volunteer soldiers.