Why was Peyton Farquhar hanged?

In
Section II of "," the scout who is posing as a Confederate soldier tells Peyton
Farquhar:

"The commandant has issued an order, which
is posted everywhere, declaring that any civilian caught interfering with the railroad, its
bridges, tunnels, or trains will be summarily hanged. I saw the order."

The Union officers know they are in enemy territory. There were
many Southern civilians like Farquhar who were trying to help the Confederate cause by
committing sabotage or even bushwhacking Union troops. That is why the order is posted
everywhere. The South was a huge place and the Union forces were surrounded by hostile
civilians. The commandant posted the order quoted by the scout because he had good reason to
fear attacks from self-appointed guerrillas, all of whom owned rifles and hated the
invaders.

Peyton Farquhar just happened to be one of the Southern activists
who got caught. He was hanged from the bridge mainly to set an example. No doubt his body was
left hanging for a long time, so that other Southerners would see it and be frightened. A
hanging body sends an eloquent message.