West view of the Ara Pacis from the Field of Mars
History
"When I returned to Rome from Gaul and from Spain, in the consulship of Tiberius Nero and Publius Quintilio, having brought to a satisfactory finish my works in these provinces, the Senate decreed that there should be consecrated in the Field of Mars an altar to the Augustan Peace and ordered that the officials, priests and vestal virgins should celebrate a sacrifice at it every year."
It is with these words that Augustus, in his spiritual testimony, the Res Gestae, tells us of the Senate's decision to construct an altar to Peace, following the conclusion of his labours North of the Alps from 16 to 13 B.C., subjecting the Reti and the Vindelici, establishing definitive control over the Alpine passes, and visiting Spain, finally at peace, founding new colonies and imposing new tributes.
The ceremonial dedication of the Altar of Peace, took place on the 30th January in the year 9 B.C.
It seems, according to the evidence provided by the historian Cassius Dione (LIV, 25.3), that at first the Senate had planned to build an altar within their own building, the Curia, but the idea was not followed through and the northernmost part of the Field of Mars, which had recently been urbanised, was chosen instead. The altar dedicated to peace came, therefore, and not by chance, to be built in the middle of a vast plain, on which, traditionally, the manoeuvres of the infantry and the cavalry took place, and, in more recent times, the gymnastic exercises of the Roman youth.