For a discussion of this idea in the context of the early Roman fetial
practice, see JOHN RICH, DECLARING WAR IN THE ROMAN REPUBLIC IN THE PERIOD OF TRANSMARINE EXPANSION (1976); ALAN WATSON, INTERNATIONAL LAW IN ARCHAIC ROME: WAR AND RELIGION (1993); Thomas Wiedemann, The Fetiales
: A Reconsideration, 36 CLASSICAL Q.
103) When the early Romans wished to establish a treaty with a foreign people, a fetial called the verbenarius, or vervain bearer, traveled to the other party's territory carrying a piece of vervain, an herb native to the Capitoline, with the soil from the sacred precinct still attached.
109) If after thirty-three days no just reparation had been made, the fetial would return to Rome, where the Senate would deliberate and vote to declare war.
The fetial code and the elaborate rituals accompanying such oaths attest to their privileged status in Roman politics and religion.
Where no oaths bound Romans to behave in a certain way, they were free to act as they pleased--even barbarously by modern standards, so long as their conduct complied with the fetial code of just warfare.
Dionysius reports that the fetial tradition had been linked to both the Rutulian city of Ardea and the Aequicoli, an Italic tribe.
Beyond the purview of the fetials and their ancient code, the sanctity of oaths found equally powerful expression in Rome's other interaction with her enemies and her allies.