explain why might a historian might not think that joseph f greens letter reflects the morale of the entire union army

Battle of Fredericksburg was devastating for the Union Army, so the letter may not be a proper representative.

Explanation:

The battle of Fredericksburg was a bad routing of the Union Army where heavy looses were incurred. The letter written by Joseph F. Green at the time was unusually optimistic.

One would believe that the greatest loss of Union Army during the Civil War would stir greater sadness in the solider.

So either he was isolated from the thick of action and did not realize that it was as bad as it actually was or that he had a vested interest in portraying the situation as better than it was.