Original: 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.
Simplified: 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.