Whether you knew someone in a war, or just came back from a war, war affects everyone. Jack was a man who fought in World War 1. He was injured in the war. He got a scar on his groin which enabled him to have sex. Brett, the woman he loved lost her lover in the war. Because of the fact that he could not have sex Brett did not want to be with him. Also, she lost her lover in the war and even though she loved Jack a lot she did not want to be with him for that reason. .
Jake feels like less of a man because he basically had no manhood. He lost it in the war. Brett did not want to be with him because his penis was useless. "Oh, Jake, "Brett said we could have had such a damned good time together."" Brett says this quote at the end of the novel. She is saying that if it was not for his injury they would have been the perfect match. Also, Mike Campbell, Brett's fiancé is always in debt, he drinks a lot, just like Jake does. .
The war causes Brett to have multiple lovers. She had lost the man she loved in the war and this was her way of dealing with it. It also cause her to have a stalker. Robert Cohn is Jake Barnes friend who follows Brett everywhere because he thinks he falls in love with her. Jake asks Brett if they could just live together and she responds, "I don't think so. I'd just tromper you with everybody." Tromper means to commit adultery. The reason that she knows she will cheat on him is because she is going to want sex and he cannot give it to her. All she wants is sex. It is what she enjoys.
War messes with everyones' heads. Jake is confused about his manhood, along with the other veterans. It makes Brett want to have more than one lover. It scars Jake for life and takes away what his one love really enjoys. It left people in debt and it altered everyones' lives, whether they were involved or not.