Factual error: In the beginning when seen the Union attacking, the American flag has 50 stars not the 36 at the time of the war. The flag is incorrect.
Suggested correction: First, it's not a 50-star flag. There's a clear shot of the flag in one scene where you see it has 3 rows of 7-stars and 2 rows of 6-stars for 33-stars, although not in a pattern I'm familiar with. There's another shot where it's less clear, but you still only see 5 rows of stars and not 9 offset rows. Second, Nevada was the 36th state and didn't join until October 1864. The 36-star flag wouldn't have come out until July 4th, 1865, which was after the Civil War and wouldn't have been flown during the war.