Bellona was the Roman goddess of war and the companion of Mars; sometimes she was called his sister, sometimes his wife, sometimes his charioteer. She was an old indigenous deity, worshipped in Italy long before the Greek Enyo with whom she was later equated, and her name comes from the Latin bellum, war. In her temple beyond the pomerium of the city, on the Campus Martius near the Circus Flaminius, the Senate received foreign ambassadors and generals returning to claim a triumph; for the temple lay outside the sacred bounds of Rome, and within those bounds neither armed men nor foreigners might tread.

Before this temple stood the columna bellica, the column of war, and at its foot a small piece of ground was held to be foreign soil — a fiction of law, by which the priests of Bellona could declare war upon any enemy without leaving Rome. The fetial priest hurled a spear over the column into this ceremonial "enemy ground," and the war was thereby formally begun. Even when Rome's wars were carried far across the sea and a real frontier was no longer accessible, this rite was preserved.

Bellona's worship in the late Republic and early Empire became increasingly entangled with the cult of the Cappadocian goddess Ma, whose ecstatic priests had been brought to Rome by Sulla after his eastern campaign; the bellonarii of the Imperial period gashed themselves with axes and double-edged knives in the goddess's processions, sprinkling the blood upon her image. The older Roman Bellona, Mars's stately companion, gave way before this newer and more lurid figure — and the moralists of the Empire deplored the change.