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.
Scipio Africanus: One of History's Greatest Generals 
It is interesting how the Second Punic War figures into Rome’s usually-expansionist policy. Goldsworthy, in “The Fall of Carthage” makes it clear that however defensive the war may have become, Rome desired a subsequent conflict with Carthage to check their expansion in Iberia. Before World War II, particularly 2,000 years before World War II, the legal principle of “Rights of Conquest” was as accepted as a rule of war as certainly as the Geneva Conventions are today.
And let us not indulge Empire bashing against Rome: Carthage was no more virtuous (and indeed, quite less compared to Scipio himself) when it came to “stabbing someone and taking their stuff.”
It would be wise for humanity to drop the pretense of defense when the intention is genuinely expansionist: not from an anti-war perspective, but from a “we should be smart” perspective. Any nation that wants to make war on another nation will find an excuse, no matter how justified or “not justified.” As was stated by Abraham Lincoln when he held back on the Emancipation Proclamation until after victory at Antietam and was shown by the ineffectiveness of the League of Nations: high-sounding principles are effectively worthless unless backed by force of arms. As Cicero said, “In time of war, the law falls silent.”
In spite of its treaties, it was Rome’s legions that kept the peace after their wars. Napoleon only backed down when it was clear Wellington was advancing from Spain and the rest of the Coalition was advancing from Germany. Hindenburg and Ludendorf refused to surrender until Germany’s military situation was truly dire, and even Saddam Hussein laughed at U.N. Resolutions just as Hitler lied to Chamberlain’s face until met with force.
I do find it odd that Caesar’s conquest of Gaul is viewed by even some neo-progressive historians as at least marginally “heroic.” Caesar’s invasion of Gaul was almost as nakedly “stab you and take your stuff” as Crassus invasion of Parthia. Hannibal Barca’s invasion of Italy (and his troops who intended to plunder, murder, rape and enslave) is revered as sacred by elements of the black nationalist community. But Cortez’s conquest of the Aztecs is viewed with utter contempt. Militarily, the odds were quite against Cortez, even with the advantage of firearms: at most, he could muster a few thousand infantry against over a quarter-million Aztec warriors. It took no small amount of diplomacy for Cortez to secure the Tlaxcallans as allies. Napoleon is treated as a sacred hero as he conquered Europe with an army whose conduct was utterly criminal (burning civilians alive in Portugual and Spain, and did even worse in Russia) but Wellington, who went out of his way to treat civilians humanely while fighting a defensive war with the support of the Spanish people, is rarely looked upon as such.
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