The Battle of Philippi was the final battle in the Liberators' civil war between the forces of Mark Antony and Octavian and the leaders of Julius Caesar's assassination, Brutus and Cassius, in 42 BC, at Philippi in Macedonia. The Second Triumvirate declared the civil war ostensibly to avenge Julius Caesar's assassination in 44 BC, but the underlying cause was a long-brewing conflict between the Optimates and the Populares.