The Mexican-American War, which took place from 1846 to 1848, marked the United States first battle on foreign soil. Under the presidency of James K. Polk, the 11th president, America would expand more than one-third (History.com).
The main causes of the Mexican – American War was disputes of land. President James K. Polk belied in the idea of the Manifest Destiny, and ideal that America not only  had the God given right to occupy and civilize North America, but was destined to. He had his sights set on what is known today as the American Southwest; California, Utah, Nevada, Arizona, New Mexico, and Western Colorado (Mt. Holyoke College). 
After an offer from President Polk to buy the land along California and New Mexico was rejected, Polk proceeded to place troops along the Rio Grande and the Neuces River, therefore instigating the first battle of the Mexican American War (Gordon). 
The first attack broke out on April 25th, 1846, on General Zachary Taylor and his soldiers, killing about a sixteen people (Stevenson). 
After two years of battle, on February 2, 1848, the Treaty of Guadalupe Hidalgo established the Rio Grande as the United States- American border. Mexico also recognized that the United States had possession of Texas, and sold California and the rest of the Northern Territory of the Rio Grande for $15 million (Stevenson).