Answer: an encyclopedia article about the Civil War
Explanation:
An encyclopedia article is always the best source when trying to achieve a credible and accurate account of a historical event.
Letters from soldiers fighting in the Civil War might provide great insight, but their records will be quite subjective. The same happens with a user-uploaded video reenactment. A wiki user-generated content is not trustworthy enough. And the map might be useful but require further investigation to be used in the report.