If you look at the heritage of prominent leaders on both sides you will see they are mainly of Celtic descent.(There are 7 Celtic Nations)
Jefferson Davis (Welsh),George Henry Thomas (Welsh),Stonewall Jackson (Ulster Scots),U.S. Grant (Ulster Scots),Patrick Cleburne (Irish), Phil Sheridan (Irish).
Reputedly, the South's white population was 75% Celtic at the outbreak of the A.C.W.
There is a very good documentary available, "How the Scots-Irish shaped America", it is narrated by Senator James Webb, there is a lengthy segment on the A.C.W.