In April of 1846, the Mexicans attacked Taylor's men but were defeated, even though they outnumbered U.S. troops 4-to-1. In May of 1846 Congress declared war on Mexico.

Taylor then invaded Mexico capturing the cities of Matamoros and Monterrey. In February 1847, General Santa Anna led 20,000 Mexican troops in the Battle of Buena Vista against Taylor' 5,000 men. General Taylor' men were victorious. General Winfield Scott was sent to capture Mexico City. In September 1847, his men occupied Mexico City.

A peace treaty was signed in February 1848 at Guadalupe Hidalgo. The treaty recognized the U.S. annexation of Texas and ceded the California and New Mexico territories to the United States, making up all of the states in the Southwest.

Illinois played an important role in the Mexican War. Congress authorized President Polk to accept 50,000 volunteers. The Illinois governor issued a call for 30 companies of 80 men in each company. To this request, 75 companies volunteered, totaling some 6,000 men. Of these, 30 companies were selected and the others remained at home.