Since Antigone is not the first story in the Oedipus trilogy and it describes the events happening after the war. However, there are some details about the death of the previous rulers. Polynices and Eteocles were brothers who did not manage to share the throne and killed each other in the battle.
There is a short explanation of what happened before and during the war for Thebes in Antigone. Most likely it is there to help and fill in the audience who are not familiar with the story. Antigone‘s plot portrays the aftermath of the battle and the tragic death of the main characters. It appears that it all started with Oedipus, who married his mother and killed his father. Eventually, he was thrown out of the city, and his two sons, Eteocles and Polynices, are left to rule. However, they did not manage to share the throne.
While Eteocles stayed there with the help of Creon, Polynices was forced to leave and seek help elsewhere. After creating an army, the exiled brother came back to win Thebes with force. However, the protectors appeared to be too strong, and the city did not fall. Polynices met with Eteocles at one of the gates for a fight, and they ended up killing each other. Antigone‘s timeline begins the day after the battle when Eteocles is already buried with honors as a hero. Creon does not allow to do the same for the other brother, thinking of him as a traitor. That decision triggers a series of tragic suicides throughout the play.