Why did Octavian cooperate with the Roman Senate against Mark Antony at first?

After the assassination of Julius Caesar, Octavian (the future Augustus) cooperated with the Roman Senate against Mark Antony primarily as a strategic political maneuver to establish himself as a legitimate leader.12

Following Caesar's death, Antony had seized control of the city, the official machinery of government, and Caesar's personal assets. Octavian, only 19 years old at the time, sought to challenge Antony for the leadership of the Caesarian faction. By aligning with the Senate—which, under the influence of Cicero, was actively opposing Antony—Octavian gained official recognition and military support.132

This cooperation served several purposes:
* Legitimacy: It allowed Octavian to act within the framework of the state, eventually securing the rank of senator and later the consulship.2
* Weakening a Rival: Cicero hoped that by using Octavian to defeat Antony, he could fragment and ultimately neutralize the Caesarian party.1
* Power Consolidation: Octavian used the Senate’s resources to build his own military strength, eventually turning the tide to his advantage before forming the Second Triumvirate with Antony and Lepidus.132