The 1st and 2nd regiments of Illinois volunteers enlisted at Alton, Illinois, in June 1846. They were put on transports bound for New Orleans, with General John E. Wool in command. They arrived in late July 1846 and reshipped for Port Lavaca, then marched to San Antonio reaching there in late August. General Wool gathered troops in San Antonio during August and September 1846 in preparation for a march into northern Mexico to Chihuahua. The march began on September 29, 1846. Because he could transport no provisions for them, Wool left behind two regiments of Illinois Volunteers (about 600 men) with instructions to follow the main force later. The army arrived in Monclova, Mexico, on October 30 and within 2 weeks, the two Illinois regiments arrived. Monclova was taken by force on October 30, 1846. Wool’s forces stayed there a month and then began the march to Parras when he learned that Santa Anna was heading to Potosi. Wool left a force of 250 men from the Second Illinois Regiment to guard Monclova and headed out to pursue the Mexican army. The battle of Buena Vista was fought in a narrow mountain pass measuring 1 ½- 4 miles in width. General Wool arrived there with fewer than 3,000 men. His forces included the First and part of the Second Illinois. Santa Anna arrived with 20,000 men. It was here that Colonel Hardin and Colonel Clay were lost. The units were saved by Colonel Bissell’s quick actions and Washington’s Battery as they opened fire upon the enemy allowing for their escape. The First and Second Illinois went on to fight the rest of the battle. The next day Santa Anna retreated to Agua Nueva and ended hostilities in the area of Mexico along the Rio Grande. The First and Second Illinois lost 91 men and counted 85 wounded. The troops were mustered out of service on June 17, 1847.