After Caesar's assassination, Mark Antony sends a Servant to Brutus bringing the message that Mark Antony pledges his allegiance with Brutus and "will follow the fortunes and affairs of noble Brutus" (3.1.149-150). All that Mark Antony says he craves is an explanation for the Conspirators' actions. Antony then meets with the Conspirators and shakes their bloody hands, and proclaims, "Friends am I with you all and love you all"(3.1.241).Unfortunately for Brutus, Antony's loyalty is a mask and Brutus and the Conspirators are betrayed during Antony's famous funeral speech, "Friends, Romans, Countrymen..."
Antony makes the conspirators believe that he is with them if they can explain why Caesar was so much of a threat that he had to be killed. He even goes from man to man shaking each hand, still bloody from Caesar's assassination. Although Cassius does not buy Antony's act that he understands why they did what they did, a trusting Brutus believes that Antony has come over to their side. When Antony requests to be allowed to speak at Caesar's funeral, Brutus thinks it's a good idea to show the people that even Caesar's close friend Antony is with the conspirators. Brutus will talk to the crowd first to explain the assassination, and then Antony will be allowed to say some kind words on Caesar's behalf just as long as he doesn't say anything negative about the conspirators. It was this poor judge of character that allowed Brutus to be dragged into the conspiracy in the first place. Brutus was manipulated by Cassius, and Cassius believes that Antony is doing the same thing with regards to the oration. Unfortunately for Brutus and the rest of the conspirators, Antony is allowed to speak. His words turn the mob against Brutus, and Cassius was probably saying to himself, "I knew it!"
He wins the hearts of rome first~!