Union XII Corps

      General Joseph K. Mansfield                      General Henry Warner Slocum                General Alpheus S. Williams                     General John W. Geary



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The purpose of this site is to provide a history and resource guide to the XII Corps of the Union Army during the American Civil War.  The XII Corps was active from March 1862 until it was reorganized and designated as the XX Corps, in April 1864.

The XII Corps served as part of the United States Army in the Army of the Potomac.  It served in several major campaigns of the war, including the Battles of Antietam, Chancellorsville, Gettysburg, and Lookout Mountain.  Many illustrious officers of the Union Army served as its commanders, division and brigade leaders.  General Joseph K. Mansfield, who was mortally wounded at Antietam, was its first commander, and General Henry Warner Slocum was the second commander.  Among the division leaders were Brigadier General Alpheus S. Williams and Brigadier General John W. Geary.  Some of its noted brigade commanders were Brigadier General Thomas H. Ruger, Brigadier General Henry H. Lockwood, Brigadier General Thomas L. Kane, and Brigadier General George Sears Greene.

The XII Corps had only two divisions, instead of the normal three.  But among its regiments were some of the finest to serve during the Civil War.  A few of these regiments were: the 2nd Massachusetts; the 7th Ohio; the 13th New Jersey; the 28th and 46th Pennsylvania; the 3rd Wisconsin; and the 60th, 78th, 102nd, 137th, and 149th New York regiments.

The XII Corps played a decisive role in the defense of the Union right wing, on Culp's Hill, at Gettysburg, July 1-3, 1863.  Yet, the officers and men of the XII Corps received no Medals of Honor.  We are beginning a research process to recommend several officers and enlisted men of the XII Corps for our nation's highest honor.  Please visit our Medal of Honor Initiative page for up to date information.

We added the description of the Gettysburg campaign from Campaigns of the Civil War (vol. 6).  Visit the Gettysburg page to view a searchable text file or an image of the original document.  We also added information about the life of George Sears Greene and his contributions at Gettysburg, from the book, In Memoriam, George Sears Greene, published in 1909.

We also added the complete battle report of the XII Corps in the Gettysburg campaign.  This is in searchable text.  Included are the reports by the corps commander, division and brigade commanders, down to the regimental level.  This provides the most detailed account of the XII Corps in its finest hour.  It is from the actual text of the Official Records of the War of the Rebellion, Volume XXVII, pages 1-902.  Included is the report from General Meade, commander of the Union Army at Gettysburg.

Please click on the Bibliography page for links to full text of Civil War books and articles.

Here is a list of our favorite Links about the American Civil War.

We will be continually adding information and resources to this site.

Look forward to your comments or suggestions.

Please see our sister website on the XX Corps and the Army of Georgia: www.ArmyOfGeorgia.com.  This website deals principally with the XX Corps and General Sherman's march in the Georgia and Carolinas Campaigns, 1864-1865.

Several prominent officers of the XII Corps were abolitionists or opponents of slavery.  Among them were XII Corps commander General Henry Warner Slocum, Brigadier General John White Geary, Commander of the 2nd Division, and Colonel Thomas L. Kane, Commander of the 2nd Brigade of the 2nd Division.  Please visit our website, American Abolitionists: Conscience of the Nation, at: www.AmericanAbolitionists.com.

NEW!  Eric Saul, the editor of this website, has recently produced a new website, the Civil War Encyclopedia. The purpose of this site is to provide those interested in the American Civil War with an extensive body of historic reference works on the war. It deals with all aspects of the Civil War, including the political and social history of the war. The central part of this website is an exhaustive, encyclopedic reference document on the Civil War. This includes biographies of soldiers, political leaders, inventors, financiers, and others involved in the war effort. It also includes a list of all of the Civil War battles, arranged chronologically. Also included are military terms and concepts. Other documents will include citations from Union and Confederate Army and Navy regulations and the Articles of War.

Saul has also compiled an Encyclopedia of Civil War Biography, which can be viewed on his website AmericanAbolitionists.com.  It is a comprehensive list of notable Americans from the mid-19th Century.  This constitutes a who's who of America in politics, science, industry, and the arts.  These individuals represented both sides of the argument over slavery in the United States.  Also included is an extensive list of military and naval officers on both sides of the conflict.  For those interested in military and naval officers, Union and Confederate, we have compiled an edited version with just entries of those military personnel involved in the Civil War: Encyclopedia of Civil War Military Biography. In addition, this list contains names of prominent Union and Confederate medial officers and personnel. There are also prominent members of Civil War relief agencies, such as the Sanitary and Christian Commissions. These entries were drawn from the 1886-1889 edition of Appletons' Cyclopaedia of American Biography (6 volumes).  There are more than 3,000 individuals in this Encyclopedia.

Finally, we have created a general Civil War website that you might find informative: www.CivilWarLibrary.org.

This site was established on February 23, 2014.

Content last updated May 31, 2020