What motivates Farquhar in "An Occurrence at Owl Creek Bridge"?

Farquhar's actions are motivated by loyalty to the Old South. It sounds like Farquhar did not join the army because he is not great at taking orders—he's described as "imperious"—but he feels

No service was too humble for him to perform in the aid of the South, no adventure too perilous for him to undertake if consistent with the character of a civilian who was at heart a soldier, and who in good faith and without too much qualification assented to at least a part of the frankly villainous dictum that all is fair in love and war.

In other words, he evidently believes that all is fair in war, at least. In the second part, we learn that, one day in the recent past, a "gray-clad soldier" approached Farquhar's home and told Farquhar that the Union commandant had issued an order than anyone who attempted to interfere with the railroad would be hanged; this is an indication of just how important the railroads must be to the Northern war effort. Farquhar asks what one might be able to accomplish if he were able to overpower the sentinel guarding the railroad bridge, and the soldier tells him about a "'great quantity of driftwood'" that has pushed up against the bridge on one side; it would catch fire quickly. At the end of this section, the soldier is revealed—to the reader—to be a "Federal scout." He was only dressed as a Confederate soldier, but he is actually a Union soldier. Therefore, we can assume that Farquhar tried to carry out this plan and that the Union army was waiting for him, knowing what he would attempt because of the scout. Thus, Farquhar seems to have acted out of a desire to disrupt the Union army's success in the South and thereby help the South to win.

What motivates and frightens the protagonist in "An Occurrence at Owl Creek Bridge"?

Peyton Fahrquhar is a staunch supporter of the Confederacy during the Civil War and is devoted to the South's cause, which is why he attempts to burn down the Owl Creek Bridge after receiving information about it from a Union spy. Since Peyton is a wealthy slaveowner, he did not enlist as a soldier, but felt that no service was too humble for him to complete in order to aid the Confederacy. Peyton Fahrquhar knows that destroying the Owl Creek Bridge would significantly halt the Union's advance and learns that the Union Army has issued an order that any civilian caught interfering with the railroad or bridges will be hanged.

He also knows that there is a significant amount of dry wood near the bridge that would instantly go up in flames if ignited. As a Southern sympathizer, Peyton attempts to set the bridge on fire and aid the Confederacy's cause by temporarily stopping the Union’s advance. As a slave owner, Peyton believes that the North is diametrically opposed to his way of life and he desires to preserve the South. Peyton does not know that he was speaking to a Union spy, and his fantasies while standing with a noose around his neck reveal that he fears death.