Charlie O'Brien, the proprietor of a small traveling circus, encounters an elderly man, Jacob Jankowski, who is separated from his nursing home group. The two strike up a conversation and Jacob reveals he had a career in the circus business and was present during one of the most infamous circus disasters of all time, the 1944 Hartford circus fire and the Hagenbeck-Wallace Circus wreck.
Jankowski tells his story to O'Brien, starting in 1931 when he was a 23-year-old veterinary medicine student at Cornell. During his final exam, he is informed that his parents were killed in a car crash. His father has left huge debts, and the bank was foreclosing on Jacob's home. Feeling there is no point in returning to school, and having no home to go to, he jumps onto a passing train where he meets a man named Camel.
Jacob discovers that he jumped on the Benzini Bros. circus train. He sees a beautiful woman, Marlena Rosenbluth, and meets August, the circus's ringmaster, head animal trainer, and Marlena's husband. Jacob reveals he studied veterinary science and August hires Jacob as a vet for the circus animals after Jacob tells August that Silver has laminitis.
August instructs Jacob to fix Silver and keep him performing as long as possible. But Jacob cannot bear to see Silver's suffering and takes it upon himself to put Silver down. August is furious with Jacob's decision to euthanize Silver against orders. To show Jacob who is boss, he threatens to throw him from the moving train â telling him that an animal's suffering is nothing compared to a man's, and that Jacob must carry out all of August's future orders if he wishes to keep his job.
August eventually procures Rosie the elephant as Silver's replacement. He invites Jacob to his car for dinner and cocktails with him and Marlena. Jacob watches the married couple flirt and dance in front of him, but it becomes clear that their relationship is complicated because August is possessive, jealous and rough with Marlena.
In the next few weeks, August becomes frustrated when Rosie the elephant seems impossible to train. August is brutal with Rosie, beating her when she fails to follow orders. After a beating that August gave to Rosie when she ran away after fleeing from the event and dropping Marlena, Jacob realizes that the elephant only understands Polish commands. After that, Rosie performs beautifully and the circus enjoys a short period of success. While working together to train Rosie, Jacob and Marlena fall in love. After August discovers this, he cruelly taunts the two. Marlena discovers that August plans to throw Jacob from the train and they run away together, hiding in a local hotel. Soon after consummating their relationship, they are ambushed by August's henchmen who drag Marlena away and beat up Jacob.
Jacob returns to the circus to find Marlena. Marlena tells Jacob that his friends Walter and Camel were thrown from the train and killed. Several circus employees have become fed up with August's murderous cruelty and unleash their revenge by unlocking all the animals' cages while the big top tent is jam-packed with an audience enjoying Marlena and Rosie's performance. Jacob attempts to find Marlena in the chaos and August attacks him. When Marlena tries to stop August from beating Jacob, he turns his fury on her and chokes her, while one of August's henchmen beats Jacob. Two circus workers save Jacob and he sees Rosie hit August on the back of the head with a metal spike, killing him.
He returned to Cornell and finished his degree. He and Marlena took several of the horses and Rosie and got jobs with Ringling Bros. and Barnum & Bailey Circus with he as a veterinarian and she continuing to perform with the animals. They married and had five children and kept Rosie until her death many years later. He took a job as a vet at the Albany Zoo and Marlena died peacefully in her bed at an old age. He convinces O'Brien to hire him in the ticket booth.

Question: Why did August  perpetrate violence against Jacob?
Answer: In retaliation for the affair between Jacob and August's wife, Marlena.