The 3rd Missouri Infantry had been reduced to four companies during the process of consolidating with the 5th Missouri Infantry; these companies became Companies B, D, E, and H within the consolidated regiment. Colonel James McCown of the 5th Missouri Infantry commanded the new regiment, as Gause, commander of the 3rd Missouri Infantry, had been transferred to the Trans-Mississippi Department. The regiment fought in the Atlanta campaign in 1864, including at the Battle of New Hope Church on May 25 and at the Battle of Kennesaw Mountain on June 19. On October 5, the regiment fought at the Battle of Allatoona, where it suffered 76 casualties, and on November 30, at the Battle of Franklin, where it lost 113 of the approximately 150 remaining men. In February 1865, the regiment was transferred to Mobile, Alabama, adding to the city's defenses, where it surrendered on April 9 at the Battle of Fort Blakely.