In the rest of the text, how does the author develop this central idea that spying during the civil war still involved mostly fieldwork?

by enhancing the reader’s understanding of which technological advances were used during fieldwork
by changing the reader’s perceptions about how fieldwork could the union cause
by including more detailed information about who did fieldwork for the union and how it was done
by adding complexity to the topic by raising the moral issues involved in fieldwork and other espionage

The correct answer is "by including more detailed information about who did fieldwork for the Union and how it was done".

Espionage was a practice which was greatly used during the Civil War. The author focuses on the terms "scouts" and "spies". Armie would normally encounter outsiders who approached them. Some of them identified themselves as scouts coming from other regiments. However, if they were not to be believed, they were subsequently executed under the presumption they were "spies". On the other hand, some spies actually got away with it and ended infiltrating enemy ranks. After a couple of weeks with the enemy, they would escape and return to their squads with valuable information.