Question

What is a central idea that the author develops throughout The Dark Game?

During the Civil War, ordinary individuals who wanted to aid the war effort acted as spies.
Elizabeth Van Lew fed information to the Union Army about the movements of the Confederate Army.

Answer 1

The first statement is the central idea: that during the Civil War ordinary individuals who wanted to aid the war effort acted as spies.

Why: this idea is broad and developed throughout the book by showing multiple civilians who risked their lives to gather and pass information, describing their motives, methods, and effects on the war. The second statement (Elizabeth Van Lew fed information to the Union Army about Confederate movements) is a supporting detail — a specific example the author uses to develop that central idea.