"Meanwhile Phraates, fearing that Augustus would lead an expedition against him because he had not yet performed any of his engagements, sent back to him the standards and all the captives, with the exception of a few who in shame had destroyed themselves or, eluding detection, remained in the country. Augustus received them as if he had conquered the Parthian in a war; for he took great pride in the achievement, declaring that he had recovered without a struggle what had formerly been lost in battle. Indeed, in honour of this success he commanded that sacrifices be decreed and likewise a temple to Mars Ultor on the Capitol, in imitation of that of Jupiter Feretrius, in which to dedicate the standards; and he himself carried out both decrees. Moreover he rode into the city on horseback and was honoured with a triumphal arch.."
Cassius Dio, Roman History (LIV.8)
Spanning the gap between the Temple of Castor and the Temple of Divine Julius, the Arch of Augustus probably was dedicated in 29 BC (the same time as the Curia and Temple of Divine Julius) to celebrate the victory of Octavian at Actium two years earlier. But it also could have been an arch of 19 BC, commemorating the previous year's return of the legionary standards lost by Crassus to the Parthians in 53 BC. (This diplomatic victory also is commemorated on the cuirass that Augustus wears in the statue found at the villa of Livia at Prima Porta).
Coin reverses (this denarius is from 16 BC) depict a triple arch surmounted by a four-horse chariot, the reins held by Augustus, himself, with Parthians on the sides offering standards to the triumphator. Fragmentary lists of consuls (fasti consulares) and triumphs, if they belong to this arch, would reinforce the claim of Augustus to have restored the Republic by avenging Caesar's death at Philippi. The first triple arch in Rome, it served as the prototype for the Arch of Severus.
Although Dio speaks of a temple to Mars Ultor on the Capitoline and provincial coins of 19-18 BC do depict a small round temple or tholos displaying the recovered standards, it likely did not exist. Rather, the standards probably were deposited in the Temple of Jupiter until they could be placed in the Temple of Mars Ultor in the Forum of Augustus, which was dedicated in 2 BC. The coins, themselves, indicate that a temple was decreed, but it may be supposed that the plan was abandoned in favor of a more grand temple in the Forum of Augustus.
