Breaking the bond of trust in a relationship and deceiving another person are considered as forms of betrayal. In Shakespeare's Julius Caesar, betrayal can be regarded as the foundation of the whole story and throughout the play between other characters. Due to the anger of Cassius, the entire play deals with the betrayal of Caesar by Cassius; there are occurrences of this treachery before and after his death.
The first betrayal of Caesar can be seen at the very beginning of the play, when Flavius and Marcillus sent the commoners away then proceeded to take the scarves off of the statues celebrating Caesar. They made the comment, "These growing feathers pluck'd from Caesar's wing will make him fly an ordinary pitch," (Act 1, Scene 1). In other words, the two conspirators feel that sending away Caesar's followers will give Caesar a reality check of sort and bring his ego down.
The next example of betrayal can be seen by Cassius working to get Brutus to his side, away from believing in Caesar. He does this first by sending him a fake letter and proceeds to tell him about the reason why he is so distraught, that he feels betrayed by Caesar. He tells Brutus about a time before when they were swimming across the Tiber River and Caesar was almost drowning, calling out, "Help me, Cassius, or I will sink!" (Act 1, Scene 2). He describes how he saved Caesar's life, then tells Brutus, "and this man is now become a god, and Cassius is a wretched creature and must bend his body," (Act 1, Scene 2). This scene describes Cassius, bowing down to Caesar as a king even though he saved his life.
Most of the illustrations of betrayal in this story are fairly upfront, until Act 3, after Caesar has been killed. His friend Mark Anthony acts as though he is betraying Caesar in order to take his revenge later. When he first arrives at the murder scene, he shakes hands with all of the conspirators that have killed Caesar, even though their hands are covered with blood. He then comments, "Shall it not grieve thee dearer than thy death to see thy Anthony making his piece, shaking the bloody fingers of thy foes, most noble! In the presence of thy corpse?" (Act 3, Scene 1). He is actually speaking to the spirit of Caesar.
Anthony plays this part of fake betrayal to Caesar, continuing by agreeing to Brutus that he will not say anything bad about him after Brutus is done speaking at the funeral of Caesar. Yet the true betrayal happens when Anthony begins to discount everything Brutus has just said to the citizens, and he turns the citizens against Brutus and the other conspirators. The citizens become enraged as they feel that their leader, Caesar, has been betrayed by the killers.
The last example of betrayal can be seen as family betrays another family. In Act 5, there is a conversation between Lepidus, Octavius, and Mark Anthony. The first family betrayal is when Lepidus consents to have his brother killed along with the other conspirators. Octavius asks Lepidus, "your brother too must die; consent you Lepidus?" to which Lepidus answers, "I do consent," (Act 5, Scene 1). The ensuing act of family betrayal is when Mark Anthony agrees that his sister's son, Publius, will be killed as well. Anthony replies without hesitation, "He shall not live; look with a spot I damn him," (Act 5, Scene 1).
There are other cases of betrayal in Julius Caesar, because betrayal is a concept that the whole story is based on, interweaving almost all the characters of the story. From the major storyline of the betrayal of Caesar, to the minor betrayals between characters which cause Caesar's death ultimately makes the theme of the story.