The 25th Regiment Illinois Volunteer Infantry was organized on June 1, 1861, and mustered in Saint Louis, Missouri, into Federal service for 3 years on August 4, 1861, following President Abraham Lincoln's May 1861 call for volunteers to serve for 3 years in regiments organized by state governments.

[2] The 25th Regiment of Illinois Volunteers marched to Cape Girardeau, MO under General Jeff C. Davis after the Battle of Pea Ridge.

As a response to General Breckenridge's attack on the same flank, the 19th Illinois Volunteer Regiment moved forward, supported by the rest of the 25th.

They held the line long enough for Captain John Mendenhall's artillery to repel General Breckenridge's attack.

This action allowed the army to cross the river and engage in the Battle of Chickamauga, where the 25th would go on to lose 2/3 of their men.