News came that Brutus and Cassius were assembling an army in order to march on Rome . Antony , Octavian and Lepidus joined as allies , in November 43 BC , to stop Caesar 's assassins . When they knew that Marcus Junius Brutus and Gaius Cassius were assembling an army in order to march on Rome , Antony , Octavian and Lepidus allied together in November 43 BC , forming the Second Triumvirate to stop Caesar 's assassins .