In Julius Caesar, why does the letter convince Brutus to join the conspirators?
Brutus found the letter Cassius had Cinna leave for him, and assumed the people of Rome were pleading with him to act against Caesar. The letter convinced him there was popular support for his joining the conspiracy. He pondered it before his guests, the other conspirators, arrived.
'Speak, strike, redress!' Am I entreated
To speak and strike? O Rome, I make thee promise:
If the redress will follow, thou receivest
Thy full petition at the hand of Brutus! (Act II, Scene 1)
Brutus cared deeply about public opinion and his reputation. This was why he was easily persuaded to join the conspiracy. Cassius arranging the letters just made it easier for him to believe he was important, and people were talking about him as a leader. Brutus already had an exaggerated idea of his own importance. The letter played on that idea and convinced him to take the plunge.
Once Brutus joins the conspiracy, he does so at full-tilt. He takes complete control, not allowing Cassius any say in how things happen and not taking any advice from him. Brutus is convinced he is the one the people want, and he knows best.
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