When the reader discovers that Farquhar has been tricked by a Federalist agent, the author is using what literary device to cause the reader to...

withheld mostin Part I in order to get into the dramatic material immediately. In Part II he is
mainly providing the necessary exposition in the form of a flashback. When the reader realizes
that the disguised Confederate soldier is actually a Federal scout, this is a very subtle form
of . It is subtle because the reader already knows from reading Part I what is going to happen.
Farquhar is going to be caught and sentenced to be hanged. Nevertheless, the foreshadowing is
very emotionally effective. We feel alarmed. We feel concerned for both Farquhar and for his
innocent and vulnerable young wife, whose own life will be very seriously affected if she loses
her husband. Then in Part III, we feel very much sympathetic towards Farquhar when it appears
that he has a chance of getting out of his dire situation and making it back to his wife and his
happy home.

Since the revelation that the soldier is a Federal scout comes
after we have seen what is happening to Farquhar at Owl Creek Bridge, we get a more vivid mental
picture of what awaits him than if the foreshadowing had occurred at the beginning of the story.
The scout returns ahead of Farquhar. He warns the officers to expect a man arriving after dark
with the intention of setting fire to the bridge. The officers can set up an elaborate trap with
soldiers stationed everywhere, all with strict orders to maintain complete silence and not to
show any light until the order is given. Some soldiers would have dark lanterns. When Farquhar
got to the bridge carrying a big can of kerosene, an officer would call out something like
"Halt!"--and lights would flare on the bridge, on the bank, and even behind the
saboteur, cutting off any possibility of escape. We can only imagine this because it is not
described. And it is not described because such description is unnecessary.

So the revelation that the disguised Confederate soldier is really a Federal scout is
foreshadowing, but it is also part of a flashback, and it is also exposition. Ambrose Bierce is
playing with time throughout the story. At the end of Part I the narrator tells us:

As these thoughts, which have here to be set down in words, were
flashed into the doomed man's brain rather than evolved from it the captain nodded to the
sergeant. The sergeant stepped aside.

Everything that
happens in Part III takes place in the one, two, three seconds during which Farquhar falls and
the slack in the rope plays out. Then the noose breaks his neck and . . .

Peyton Farquhar was dead; his body, with a broken neck, swung gently
from side to side beneath the timbers of the Owl Creek bridge.

This is truly a  beautifully crafted and deservedly famous short story. Bierce calls
Farquhar "the doomed man" when the sergeant steps off the plank. We should have known
that Farquhar didn't have a chance. Perhaps the message is that none of us has a chance of
surviving. Bierce was a noted cynic and pessimist. His experiences during the Civil War
obviously had a strong influence on his outlook on life. His notorious cynicism and pessimism
are shown in his sometimes very amusing book The Devil's Dictionary (1911).
Here is a pertinent example:

HOPE, n. Desire and
expectation rolled into one.