XXIII Corps was a corps of the Union Army during the American Civil War. A corps (a word that immigrated from the French language, pronounced like English core, but originating in the Latin corpus, corporis meaning body; plural same as singular) is either: a large military unit or formation, or(ii) a regiment with a support function. ... The Union Army refers to the United States Army during the American Civil War. ... The American Civil War was fought in the United States from 1861 until 1865 between the United States â forces coming mostly from the 23 northern states of the Union â and the newly-formed Confederate States of America, which consisted of 11 southern states that had declared their secession. ...