Initially assigned to garrison the town of Lexington, Missouri, the regiment surrendered to a much larger force of pro-secession Missouri State Guard commanded by State Guard Major General Sterling Price at the Siege of Lexington in September 1861. After being paroled, General John C. Fremont (commanding the Department of Missouri) had the 23rd Illinois mustered out of service, but in December General George McClellan (now supreme commander in chief of all Union armies) had it restored.