Answer:
Intelligence was often gathered in simple ways, such as relying on fieldwork.
Step-by-step explanation:
In the book The Dark Game by Paul Janeczko contains classified spy stories about how the American government maintained its power. The stories relating to espionage and intelligence, the spy world, and the use of secret agents to gain information about other countries are all included in this book.
In the given excerpt, Paul talks about how information is gathered in the spy world. Even though photography and the telegraph have helped simplified the relay of messages and military information, the craft of spying still relies on the age-old method of gathering intelligence from "fieldwork", talking to "prisoners of war and deserters". They still form the major source of gathering information.
Thus, the central idea of the passage is that intelligence is often gathered in much simpler ways despite the use of telegraphs and photography.