You have likely heard the term “having God on our side” used in reference to justifying war or politics (see Bob Dylan’s song With God on Our Side). The ancient Romans took this god-backed approach very seriously. They would not declare war unless (ostensibly) they gave the opposing party a chance to make amends, hence winning the favor of Jupiter and Juno to declare war.
The fetiales were tasked with insuring that Rome made the proper peace overtures before they declared war, and hence retained the favor of the gods. The fetiales’ overtures insured that Rome gave at least the appearance of fighting a bellum iustum (just war) when it fought.
The fetiales were priests from the temple of Jupiter, the god of good faith. Two fetiales would cross into the enemy’s land and declare Rome’s grievances to the first person they met there, again after entering the enemy city’s gates, and once more before the territory’s rulers. If the magistrates agreed to settle the fetiales’ grievances, they would “give peace a chance.” If not, the Roman priests declared that war would commence within 33 days. If no resolution was reached during that time period, a fetial would return to the enemy border and hurtle a blood-tipped spear across its border, in essence declaring “It’s war!”
Ostensibly, the fetiales’ mission indicated that the Roman Senate sought peace before declaring war, leading some scholars to conclude that Rome’s reputation as a warlike state has been exaggerated. In some circumstances, however, the fetiales’ mission may have been a version of the “weapons of mass destruction” approach used to initiate America’s war with Iraq, a method to justify a war mission that would be deployed, regardless of the lack of evidence to justify it.
As Richard Billows notes, “Between the 340s and the 270s (BCE), the Romans engaged in a series of wars that were nominally (my italics) defensive but highly expansionist in effect.” The “just wars” of the Roman Republic led to Rome becoming masters of the Italia peninsula.
Billows, Richard. “The Hellenistic World and Roman Republic” in The Cambridge History of Greek and Roman Warfare,” pp. 303-324.