Humanities › History & Culture The Mexican War and Manifest Destiny Share Flipboard Email Print traveler1116 / Getty Images History & Culture Military History Battles & Wars Key Figures Arms & Weapons Naval Battles & Warships Aerial Battles & Aircraft Civil War French Revolution Vietnam War World War I World War II American History African American History African History Ancient History and Culture Asian History European History Genealogy Inventions Latin American History Medieval & Renaissance History The 20th Century Women's History View More By Martin Kelly History Expert M.A., History, University of Florida B.A., History, University of Florida Martin Kelly, M.A., is a history teacher and curriculum developer. He is the author of "The Everything American Presidents Book" and "Colonial Life: Government." our editorial process Martin Kelly Updated June 18, 2018 The United States went to war with Mexico in 1846. The war lasted for two years. By the end of the war, Mexico would lose almost half its territory to the U.S., including lands from Texas to California. The war was a key event in American History as it fulfilled its 'manifest destiny', encompassing land from the Atlantic Ocean to the Pacific. The Idea of Manifest Destiny In the 1840s, America was struck with the idea of manifest destiny: the belief that the country should span from the Atlantic to the Pacific Ocean. Two areas stood in America's way of achieving this: the Oregon Territory which was occupied by both Great Britain and the U.S., and western and southwestern lands which were owned by Mexico. Presidential candidate James K. Polk fully embraced manifest destiny, even running on the campaign slogan "54'40" or Fight," referring to the northern latitude line to which he believed the American portion of the Oregon Territory should span. By 1846, the Oregon issue was settled with America. Great Britain agreed to set the border at the 49th parallel, a line that still stands today as the border between the U.S. and Canada. However, the Mexican lands were considerably harder to attain. In 1845, the U.S. had admitted Texas as a pro-slavery state after it had achieved independence from Mexico in 1836. While the Texans believed that their southern border should be at the Rio Grande River, Mexico claimed it should be at the Nueces River, further north. Texas Border Dispute Turns Violent Early in 1846, President Polk sent General Zachary Taylor and American troops to protect the disputed area between the two rivers. On April 25, 1846, a Mexican cavalry unit of 2,000 men crossed the Rio Grande and ambushed an American unit of 70 men led by Captain Seth Thornton. Sixteen men were killed, and five were injured. Fifty men were taken prisoner. Polk took this as an opportunity to ask Congress to declare war against Mexico. As he stated, "But now, after reiterated menaces, Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil. She has proclaimed that hostilities have commenced and that the two nations are now at war." Two days later, on May 13, 1846, Congress declared war. However, many questioned the necessity of the war, especially northerners who feared an increase in the power of pro-slavery states. Abraham Lincoln, then the representative from Illinois, became a vocal critic of the war and argued that it was unnecessary and unwarranted. War With Mexico In May 1846, General Taylor defended the Rio Grande and then led his troops from there to Monterrey, Mexico. He was able to capture this key city in September 1846. He was then told to hold his position with only 5,000 men while General Winfield Scott would lead an attack on Mexico City. Mexican General Santa Anna took advantage of this, and on February 23, 1847, near the Buena Vista Ranch met Taylor in battle with approximately 20,000 troops. After two fierce days of fighting, Santa Anna's troops retreated. On March 9, 1847, General Winfield Scott landed at Veracruz, Mexico leading troops to invade southern Mexico. By September 1847, Mexico City fell to Scott and his troops. Meanwhile, starting in August 1846, General Stephen Kearny's troops were ordered to occupy New Mexico. He was able to take the territory without a fight. Upon his victory, his troops were divided in two so that some went to occupy California while others went to Mexico. In the meantime, Americans living in California revolted in what was called the Bear Flag Revolt. They claimed independence from Mexico and called themselves the California Republic. Treaty of Guadalupe Hidalgo The Mexican War officially ended on February 2, 1848, when America and Mexico agreed to the Treaty of Guadalupe Hidalgo. With this treaty, Mexico recognized Texas as independent and the Rio Grande as its southern border. In addition, through the Mexican Cession, America required land that included parts of present-day Arizona, California, New Mexico, Texas, Colorado, Nevada, and Utah. America's manifest destiny would be complete when in 1853, it completed the Gadsden Purchase for $10 million, an area that includes parts of New Mexico and Arizona. They were planning to use this area to complete the transcontinental railroad.