The Twelfth Infantry was mustered into the United States service for three years August 1, 1861. The regiment remained at Cairo, where it was organized, until September 5, 1861, when, with the Ninth Illinois Infantry, it moved up to and occupied Paducah, being the first Union troops there.

With the exception of occasional expeditions, the Twelfth lay at Paducah until February 5, 1862. Four companies, however, occupied Smithland, Kentucky, until January 25, 1862-Lieutenant Colonel Chetlain commanding Post. Were engaged in the demonstration against Columbus, Kentucky, forty-six miles from Paducah, during the Belmont battle. In January were engaged in a reconnoissance in force towards Fort Donelson, Tennessee.

Endured the cold and snow and hunger and fatigue of the Fort Donelson battle; formed with the Ninth and Forty-first Illinois, McArthur's Brigade, which suffered so terribly, but fought so nobly on the 15th of February. Loss, 19 killed, 53 wounded, and 10 missing.

February 22, moved to Clarksville, Tennessee, and occupied Fort Sevier. February 26, moved to Nashville. Returned to Clarksville, March 1. Mar. 6th embarked for Pittsburg Landing. Went into camp 19th.

Took part in the terrible battle of Shiloh, being engaged nearly all the time of the two days. Colonel Chetlain commanded the regiment the 6th, but he being injured by a fall from his horse, Major Hugunin commanded on the 7th. Regimental loss, 109 killed and wounded, and seven missing.

April 28th, moved with the army on Corinth; was engaged in the siege of Corinth, doing its share of picket and fatigue duty, extending saps, etc. After the evacuation the regiment was sent with General Pope in pursuit of the enemy. Lay at Booneville six days and then returned to Corinth. The regiment remained at Corinth until the middle of September. During the approach on Corinth were in Second Brigade, Second Division, Army of Tennessee. Brigadier General Thomas A. Davies, commanding division; Brigadier General R. J. Oglesby, brigade, and Colonel Chetlain, regiment.

On the 16th September moved to near Iuka. Were not engaged in the battle of Iuka. On the 19th moved to Burnsville, where it remained till October 2.

October 3 and 4, were engaged in battle of Corinth. The division (second) Gen. Davies, and the sixth division, General McArthur, fought nearly the whole rebel army. The losses were very heavy, and the fighting most desperate. On the 4th, "Powell's Battery", which we were supporting was captured by the enemy in a charge, but was almost immediately retaken. In this affair the Twelfth took a very conspicuous and brilliant part. Supported by a small part of the Fiftieth and Fifty-second Illinois Infantry, they drove the enemy from the works, capturing a stand of colors, and turned the guns of the battery on the enemy. The division lost more than half of the men that were lost during the day, the regiment losing 17 killed, 80 wounded, and 15 missing. Captain Guy C. Ward, acting major, was killed, and brigade commander General Oglesby severely wounded. Remained at Corinth until January 24, 1863 when it was sent as train guard to Hamburg and returned.

April 12. The enemy attacked Glendale, an out-post twelve miles from Corinth. The regiment, 225 strong, in command of Major Hugunin, was ordered as reinforcement. On arriving, they found that the Sixty-fourth Illinois, "Yates' Sharp Shooters", had repulsed the attack, and they returned to Corinth. The regiment was now in left wing Sixteenth Army Corps, Major General R. J. Oglesby commanding.

June 6, 1863, moved to Pocahontas, Tennessee, to guard railroad, where they remained until 29th October, when left wing being attached to Fifteenth Corps, Major General W. T. Sherman commanding, they moved as rear guard, via Corinth, Iuka, crossing Tennessee river at Eastport - at Lauderdale, Alabama, destroying an immense cotton factory - to Pulaski, Tennessee, arriving

November 12. Here left wing remained, Fifteenth Army Corps going on to Chattanooga and battle of Missionary Ridge.

November 13, went as wagon guard with Second Iowa Infantry to near Columbia, Tennessee, and returned.

November 25. Ordered to Richland Station, Nashville and Decatur Railroad, to guard railroad, where they remained about two months, doing duty, foraging, etc.

December 29. Colonel Chetlain appointed Brigadier General.

January 16, 1864. Regiment re-enlisted as veterans.

January 18. 311 men and 24 officers started for Camp Butler, Illinois, on veteran furlough. The remainder, about 90 men, remained under command of Captain J. D. Towner.

March 18. Having re-organized at Camp Fry, Chicago, Illinois, the Twelfth proceeded to Pulaski, Tennessee.

April 28. Marched to Larkinsville, and thence by rail to Chattanooga. Started on the Atlanta campaign, 9th May, and from that time till the fall of Atlanta, was actively engaged. Was in second brigade second division left wing Sixteenth Army Corps, Colonel Aug. Mersey commanding brigade; Brigadier General T. W. Sweeny commanding division; Brigadier General G. M. Dodge commanding left wing; Major General J. B. McPherson commanding department and Army of Tennessee.

The first engagement the regiment participated in was Lay's Ferry and Rome Cross Roads, 13th, 14th and 15th May, losing two killed and 21 wounded.

May 27. Assisted in repulsing a heavy night attack on Dallas.

June 2. Left the lines and swung around toward Lost Mountain and Ackworth.

June 10. Passed through Ackworth and Big Shanty.

June 27. Slightly engaged at Kenesaw, losing three men wounded.

July 4. Engaged near Nickajack creek. Crossed Chattahoochie river and was engaged in battle of July 22, Bald Knob and Decatur; losing 35 or 40 men killed and wounded.

July 28. Engaged at Ezra Church.

During August closely engaged in the siege of Atlanta; losing some 40 men killed and wounded.

August 27. Moved out from works in the movement around Atlanta, compelling its fall.

September 1. Assisted in repulsing attack of the enemy on right of Fifteenth and left of Sixteenth Army Corps. Marched to Lovejoy's, and, September 6th, returned to East Point, where division was transferred and became fourth division Fifteenth Army Corps. Sept. 26 moved to Rome, Georgia. October 4, by rail to Allatoona.

October 5. Engaged in battle of Allatoona, losing 57 killed and wounded out of 161 muskets taken into action. Returned to Rome, remained until November 11.

On the 11th of November started on the "march to the sea", passing through Kingston, Cartersville, Cassville, Allatoona, Ackworth, Big Shanty and Marietta, arriving at Atlanta,

November 15. 16th, resumed march, passed through Jonesboro, McDonough, Jackson, crossed Ocmulgee river at Seven Islands Cotton Factory, Monticello, Hillsboro, Clinton, camping near Gordon on 24th. Lost one man near Clinton, captured by Wheeler's cavalry.

November 25. Passed through Irwinton, crossed Oconee river, where some opposition was made by the enemy, on through Irwin's Cross Roads, Wrightsville and head waters of Ohoopee river, through Summerville, camping on Scull's creek, four miles from Millen 2d December.

December 3. Crossed Ogeechee river, on picket; 4th returned, marched down the river and crossed at Jencke's Bridge on the 8th. Marched to Ogeechee canal finding the enemy in force, on the 9th, four miles from the canal. They were soon flanked out of their position, leaving a "Blakesley gun" English manufacture, in our possession.

10th. With Sixty-sixth Illinois, were sent to burn railroad bridge over Little Ogeechee, but it was burned by the enemy on our approach. Until 17th the regiment was stationed in different places on the line around Savannah, when it was detailed to guard the prisoners taken at Fort McAllister and on the march and to take them to Hilton Head.

January 10, 1865. Returned to Savannah and remained to 28th, when broke camp, and marching up the Savannah river, camped as Sister's Ferry, 31st.

February 4. Crossed on 4th, and after repairing bridge and causeway crossed Coosawatchie, on the 7th; Whippy Swamp, 8th; Salkahatchie, 9th; Beaufort river bridge, 10th; South Edisto, 11th: Charleston and Augusta railroad, 12th; North Edisto, 13th; Sandy Run Post Office, 14th; 16th Saluda river, 17th, Broad river, and entered Columbia, making a march from Savannah of 177 miles.

18th and 19th February, tore up railroad, (Charleston and Columbia); 20th, marched to Muddy Springs; 21st, Longtown; 22d, Peay's ferry; 23d, crossed Waterec river; 24th, with four companies of 66th Illinois, on reconnoissance through Camden; 25th, on picket; 26th, 27th and 28th, at Lynch's Creek; March 1st, crossed Black creek; 3d, Julian creek; 4th at Cheraw, making 164 miles from Columbia.

March 6th, crossed Peedee river, passed Gum Swamp, Springfield, Laurel Hill, North Carolina, Lumber creek, Rocky Fish creek, arriving at Fayetteville, North Carolina, March 12th, from Cheraw, 73 miles.

March 14. Crossed Cape Fear river, and on the 19th arrived near Neuse river; 18th from Goldsboro, from Fayetteville, 77 miles.

March 20, 1865. Marched 9 miles and went into position in front of the enemy. 21st, lost two men wounded; 22d, moved into enemy's works.

March 24th. Entered Goldsboro, North Carolina.

Total distance marched since leaving Savannah, over 600 miles.

April 10. Broke camp and marched in pursuit of Johnson's army; Raleigh, 14th; Morrisville, 15th, where it remained until Johnson's surrender.

April 21. Returned to Raleigh; 29th, started northward. Petersburg, May 6th, making 186 miles in six and a half days. May 9th, Manchester, Alexandria, 19th. Passed in the grand review at Washington, May 24th.

June 3d, left Washington; June 6th, arrived at Louisville, Kentucky.

July 10th, 1865. Mustered out, at Louisville, Kentucky. July 18th, received final pay and discharge at Camp Butler, Illinois.