Military records are most accessible if you know the county and regiment of the ancestor of interest. This can also help you distinguish between the Philip Moore who fathered your Great Uncle Jed, and the Philip Moore who lived across the state of Ohio from Uncle Jed's place. At the National Archives, alphabetical indices are available for many of the records, but it again presents the problem of distinguishing between men of the same name who lived in the same state during the conflict. Allowing yourself to search by county and regiment will simplify the search into military records of the Civil War.
Determining your ancestor's county of origin can often be done with family sources. Many families have kept the discharge certificate - called a D214 - of their ancestor and have it stored at home. Other accessible records like the 1860 and 1870 federal censuses can help you locate the county and state of residence shortly before and after the war.
Finding the regiment of an ancestor may require a little more searching. Muster rolls - records of those who were in attendance - can be a great place to start. Muster rolls are kept by company and are often indexed. Wounded men being treated at a hospital would have been recorded on the hospital or clinic muster roll.
Another great resource is State Adjutant General reports. All Adjutant General reports are kept at the National Archives, and many are microfilmed and housed at the Family History Library. The every-name listing in the reports will lead you to the serviceman's regiment.
An easily accessible census source for finding the regiment of a Civil War soldier is the 1890 Veterans Schedule. The 1890 census was almost completely destroyed, but the Veterans Schedule lists each living veteran, rank, company, regiment, dates of enlistment and length of service. The schedule even lists living widows of Civil War veterans.
Finding your Civil War ancestor in the applicable military records can hold a treasure chest of information, some of which may not be found in other sources. And by knowing the soldier's county of residence and regiment, the search can be a lot easier.
For more information:
See the National Archives (NARA) website at www.archives.gov or:
Guide to Genealogical Research in the National Archives. Third Edition. Washington DC: National Archives and Records Administration, 2001.
Neagles, James C. U.S. Military Records: A Guide to Federal & State Sources. Salt Lake City: Ancestry, Inc., 1994.