virginia
Maryland
Montana was not a State at the time of the Civil War.
The causes of the civil war were complex. Not just one state caused it
The address of the Association For The Preservation Of Civil War Sites is: 1140 Professional Ct, Hagerstown, MD 21740-5852
Pennsylva
virginia
Virginia saw the most (122) battles of the US Civil War.
Virginia
Maryland
You might want to break that down by state since there were approx 700,000 casualties in the Civil War. If you google search on a state + civil war (like Minnesota Civil War) you will find a listing of those who fought and died in the war. The Fed sites just take too long to drill down into to get the info you want, picking a state would make your search easier.
Montana was not a State at the time of the Civil War.
Before the Civil War most Americans felt that their primary loyalties and citizenship rest with individual state governments.
Most states have civil war records online. Some state records were not maintained for a variety of reasons after the war, so are not very complete. A sample state database is linked to this answer, but there are about 30 more.
The most popular tourist sites in Harrisburg PA are Pennsylvania State Capitol, National Civil War Museum and Metro Bank Park, they have all been given 5 stars or more to visit.
The causes of the civil war were complex. Not just one state caused it
virginia