After the war Federal (I really dislike the word "union") dead were collected from different burial sites and placed in National cemeteries. Federal authorities believed that graves of their soldiers would be desecrated at worst and at best neglected where they were. For those reasons they needed to be collected and placed in "hallowed ground." If I'm not mistaken, Andersonville would be the nearest U.S. cemetery. There are others where U.S. dead from the Atlanta Campaign were buried.