Your answer is D Chickamauga it was known for the second highest causalities at 30,000 casualties also its not B because Gettysburg was a war in the Civil War but they didn't have the much deaths to put them to the top it wasn't B because Atlanta was a campaign also it want a war in the Civil War and when it came down to rather C, or D I choose D because once again the "Savannah" wasn't a war in the Civil War and it also didn't have that much causalities or deaths as the others. Your answer is D.
