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 help 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