Caesar's assassination creates an immediate power vacuum in Rome. Antony moves swiftly to grab power and consolidate it for himself. After cleverly turning the crowd against Brutus in his famous funeral oration, Antony takes additional action to achieve his ends.
When the crowd pours into the streets in pursuit of Brutus and the others, Antony learns that Octavius, Caesar’s nephew, has already arrived in the city. Octavius, along with Lepidus, await Antony at Caesar’s house. Antony replies that Octavius has “come upon a wish,” meaning that Octavius has arrived at the exact time when Antony wants him in Rome. This passage implies that Antony has planned ahead, sending for Octavius and Lepidus prior to Caesar's funeral.
When they meet, Antony, Octavius, and Lepidus immediately condemn to death many Roman senators they perceive to be dangerous to them. They are cold, calculating, and relentless in choosing those to eliminate. Antony ensures that no Roman senator will gain power as a result of Caesar’s assassination.
Antony aligns himself with Octavius because he needs Octavius’ army to pursue the forces of Brutus and Cassius, now in retreat in Greece. By joining forces with Octavius, Antony is able to defeat Brutus and Cassius, forcing them both to commit suicide. Thus Antony, even though he must share power to some extent, manages to fill the power vacuum created by Caesar’s assassination. He determines its outcome.
The most important way that Antony affected the outcome of the assassination of Julius Caeser was by manipulating the crowd in his speech so that they were outraged at the actions of Brutus and the other conspirators. The Roman citizens were understandably upset at Caesar's death but after Brutus spoke to them and explained his reasons, the crowd supported his actions. Because they thought so highly of Brutus, they were willing to believe that his actions were honorable and necessary. It was only when Antony spoke and convinced them otherwise that they wreaked havoc on the conspirators. Had Antony not been allowed to speak, the outcome of the assassination would have been very different. The crowd would have supported, protected and defended Brutus rather than turning on him. Antony was instrumental in seeking revenge for Caesar's death.