What important battle does Marc Antony lose that seals his fate?

Marc Antony loses the Battle of Actium in 31 BCE, a decisive naval confrontation against Octavian's forces. This defeat significantly diminishes his power and influence, leading to his eventual retreat to Egypt. The loss ultimately seals his fate, as he is later defeated on land and commits suicide in 30 BCE, marking the end of the Roman Republic and the rise of the Roman Empire under Octavian.