12th Regiment, Missouri Infantry (Union)
This regiment was organized at St. Louis, Mo., August, 1861. It was attached to Fremont's Army of the West to January, 1862. 2nd Brigade, Army of Southwest Missouri, to February, 1862. 2nd Brigade, 1st Division, Army of Southwest Missouri, to May, 1862. 3rd Division, Army of Southwest Missouri, to July, 1862. District of Eastern Arkansas, Dept. of Missouri, to December, 1862. 1st Brigade, 11th Division, Right Wing 13th Army Corps (Old), Dept. of the Tennessee, to December, 1862. 2nd Brigade, 4th Division, Sherman's Yazoo Expedition, to January, 1863. 2nd Brigade, 1st Division, to September, 1863. 1st Brigade, 1st Division, 15th Army Corps, to December, 1863. 3rd Brigade, 1st Division, 15th Army Corps, to November, 1864. 
The 12th Regiment, Volunteer Missouri Infantry was enrolled between 7 August 1861 and 1 September 1861. They were ordered into service between 13 August 1861 and 19 December 1861. The Regiment was discharged on 12 August 1864 thru 19 December 1864. The Colonel's ín charge were Colonel Peter J. Osterhaus and Colonel Hugo Wangelin 
Companies in this Regiment with the Counties of Origin
Men often enlisted in a company recruited in the counties where they lived though not always. After many battles,companies might be combined because so many men were killed or wounded. However if you are unsure which company your ancestor was in, try the company recruited in his county first.
Company A - Captain Albert Afflock, Captain Herman Grendzenburg and Captain Jacob Kuercher -St. Lewis, Missour.
Company B - Captain Casimier Andle and captain Fred G. Ledergerber - Belleville, St Clair, Illinois.
Company C - Captain Herman Bendel, captain William Bechtel, Captain William Mittman and Captain Henry Seibel - St Louis, Missouri.
Company D - Captain Fred Afflock, Captain Adam Ranft and Captain Julius Sauer - St Louis, Missouri.
Compnay E - Campain Charles Demming (Denny) and Captain Henry A. Kircher - St Louis, Missouri.
Company F - Captain John Kuyser, captain Gustavus Lightfood and Captain Fred Meier - St Louis, Missouri.
Company G - Captain Anthony Engleman and Captain John Monkinhaupt - St. Louis, Missouri.
Company H - Captain William McKenzie, Captain William Schuuter and Captain Anton Steffens - St Louis, Missouri.
Company I - Captain John Ahlefild, captain Theodore, Hormann and Captan Joseph Ledergerber - St Louis, Missouri.
Company K - Caption Otto Ferdinand Stunberg and Captain Constantine Von Haesseler - St Louis, Missouri.
The information about the companies and counties is from Kenneth E. Weant's book, Civil War Records, Missouri Volunteer Infantry, Volume 3, Three Year Volunteer Regiments 11-18,25, (Arlington, Texas: K.E. Weant, c2011), pages 16-35. FHL 977.8 M2wkv v. 2
- Beginning United States Civil War Research gives steps for finding information about a Civil War soldier or sailor. It covers the major records that should be used. Additional records are described in 'Missouri in the Civil War' and 'United States Civil War, 1861 to 1865' (see below).
- National Park Service, The Civil War Soldiers and Sailors System, is searchable by soldier's name and state. It contains basic facts about soldiers on both sides of the Civil War, a list of regiments, descriptions of significant battles, sources of the information, and suggestions for where to find additional information.
- Missouri in the Civil War describes many Confederate and Union sources, specifically for Missouri, and how to find them.. These include compiled service records, pension records, rosters, cemetery records, Internet databases, published books, etc.
- United States Civil War, 1861 to 1865 describes and explains United States and Confederate States records, rather than state records, and how to find them. These include veterans’ censuses, compiled service records, pension records, rosters, cemetery records, Internet databases, published books, etc.
- Ponder, Jerry (Jerry Wayne), 1937-. The 9th Missouri Infantry Regiment C.S.A and the 12th Missouri Infantry Regiment, C.S.A. (Doniphan, Missouri : Ponder Books, c1996), FHL book 977.8 M2po.