What happens at the end of Act I Scene III to move the conspiracy plot forward?
At the end of Act 1, Scene 3 in Julius Caesar, Cassius sends letters to Brutus by way of Cinna. Cassius forged these letters to make them seem as if they were written by Roman citizens who do not want Caesar to use his power to rule Rome. This scene is a significant one, because the letters ultimately prove to be the catalyst that drive Brutus to forsake his friendship with Caesar to protect Rome's republic. More importantly, once Brutus reads the letters and resolves to join the conspiracy plotting against Caesar, we learn an important fact about his character: ultimately, he is driven by his noble desire to protect Rome, rather than by private ambition. As such, Brutus becomes a tragic figure, whose rigid sense of honor and naive gullibility lead him to accept the forged letters without question, join the conspiracy, and ultimately take his own life at the end of the play. Thus, this short moment in Act 1, Scene 3 can be seen as the driving force behind much of the play's later drama.
At the end of Act I Scene III, Cassius gives Cinna a collection of letters that he has written. They are addressed to Brutus and are made to sound as if they have come from concerned citizens of Rome pleading for Brutus to prevent Caesar from abusing his power. Cassius writes these letters to convince Brutus that the people of Rome need his help. He knows that Brutus's altruistic nature will lead him to join the conspiracy once he hears that he is needed. Cinna is told to leave the letters where Brutus will find them. Once Brutus joins the conspiracy in the next scene, their mission is secured and ready to be carried out.