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.