Final answer:
The alliance between Octavian and Antony collapsed due to their rivalries for power and a romantic alliance between Antony and Cleopatra, which threatened Rome in the eyes of Octavian and the Romans.
Step-by-step explanation:
After defeating the Roman Republic's senators and avenging the death of his adoptive father, Julius Caesar, Octavian joined forces with Antony and Lepidus to form the Second Triumvirate. However, their alliance soon began to crumble as Octavian and Antony each sought to become the dominant ruler of Rome.
The final straw came when Antony formed a political and romantic alliance with Cleopatra, the queen of Egypt, which raised concerns among Romans about Antony's loyalty to Rome. This ultimately led to their military confrontation in the Battle of Actium, where Octavian emerged victorious and Antony and Cleopatra took their own lives.