The remains of the Temple of Mars Ultor (Mars, the Avenger) stand in what was once the Forum of Augustus. The building of the temple was the result of a pledge that Octavian, the great-nephew of Julius Caesar, had made at the Battle of Philippi in 42 BCE. After winning the battle, in which Brutus and Cassius, two of Caesar's assassins, were killed, Octavian deemed he had avenged the assassination of his adoptive father. The temple was inaugurated forty years later in 2 BCE.
The huge marble statue of Mars 'the Avenger', which stood in the temple was surrounded by the legionary standards that had been lost to the Parthians in 53 BCE, but which were later recovered by Augustus in a bloodless show of diplomacy. The temple was the ceremonial focus of military politics and foreign policy. Augustus decreed that the Senate should meet there when discussing wars. Military commanders when setting off for the provinces officially took their leave there and it was also the setting for the ceremony in which high-born Roman boys (around the age of 16) assumed the adult toga, thus becoming eligible for military service.
Most of the temple has been destroyed, but three fluted marble columns (18 m high) have survived, thanks to the fact that they were once used as a support for the bell tower of Santa Maria Annunziata ai Monti, a small church which was built in the ruins. The 16th century church, which was known as the Annunziatina, on account of its small size, belonged to a convent of Dominican nuns. Although both church and convent were knocked down during excavation work in the 1920s, traces of their existence can still be seen on the ancient masonry.