 On September 20, while the division was advancing to the support of General Thomas, it became heavily engaged, and captured prisoners from four different rebel divisions.The 21st was in the hottest of the fight, behaved with great courage, never yielding except when overcome by immense odds, but after a brave but fruitless effort against a perfect torrent of the enemy was compelled to give way.After the battle of Chickamauga the regiment, in command of Lieutenant Colonel S. K. Bishop, was detached from its brigade by order of General Thomas, and was placed under General Smith, Chief Engineer of the Department, and performed duty as engineer troops, forming part of Engineer Brigade, and was on that duty during the engagement of Mission Ridge.