In September 1863, the Union army in Tennessee was defeated at the Battle of Chickamauga and two corps were sent west to help them as they were besieged in Chattanooga—the XI and XII Corps.
I september 1863 blev Unionens hær i Tennessee besejret i slaget ved Chickamauga og to korps blev sendt af sted for at hjælpe dem da de blev belejret i Chattanooga—XI og XII Korps.