The Temple of Mars Ultor (Mars the Avenger) occupied the centre of the eastern side of the Forum of Augustus. It was dedicated by Octavian Augustus in 42 B.C. on the eve of the battle of Philippi fought together with Mark Antony against Brutus and Cassius to avenge the assassination of Julius Caesar, his uncle and adoptive father. The temple was inaugurated in the year 2 B.C.

Ancient reliefs display the fronton which bore in the center the statues of Mars and Venus as the ancestors of the Romans and the Iulia family, to which both Julius Caesar and Octavian belonged. To the sides of the divine couple there were the personifications of Fortune, Rome, Palatine, the Tiber and the image of Romulus, the mythical founder of Rome in 753 B.C. and its first king.

The significance of this group lies in the legitimization of power for the Iulia family who, according to the legend, descended from Iulus Ascanius, son of Aeneas, son of Venus. Then, from Iulus Ascanius descended Rhea Silvia, who had conceived Romulus with Mars.

The Temple was completely destroyed around the times of the Ostrogoth king Theodoric (493-526) in order to reuse the marble and other construction materials. They were spared only the three columns still standing and the podium. In 9th and 10th century on the podium settled the very first nucleus of the monastery of St. Basil. Between the end of the 12th century and the beginning of the 13th, the monastery became the property of the Knights of St. John of Jerusalem (later called of Rhodes, today Knights of Malta).

In 1566 the complex was donated by Pope Pius V (1566-1572) to a group of Dominican nuns: they were built a new monastery and a new church, which occupied the area of the Temple of Mars and was dedicated to the Virgin of the Annunciation. Most of these structures has been demolished in 1924-1932 to recover the Roman ruins. Many fragments of sculptures found at the time are on display at the Museum of the Imperial Fora.