 The incomplete forum and its temple were inaugurated in 2 BC, 40 years after they were first vowed.The triumvir Octavian vowed to build a temple honoring Mars, the Roman God of War, during the battle of Philippi in 42 BC.After winning the battle, with the help of Mark Antony and Lepidus, Octavian had avenged the assassination of his adoptive father Julius Caesar.