Religious justification for war was a part of Zoroastrianism from the very beginning, c. 1200-1000 BCE. The Gathas, the part of the Avestan scripture written in the Old Avestan language and generally attributed to the prophet Zarathustra himself, or at least a small group of his immediate followers, make many allusions to violent conflict between the earliest Zoroastrians and the "Kavis and Karapans." Kavi is an early Indo-Iranian title that can mean both "religious ruler" and "divinely inspired poet" in the Avesta, and Karapan was an early Iranian priestly title, used in the Avesta to indicate those who led worship of the Daivas, the false gods and demons of Zoroastrianism.

Neither warfare nor storytelling are important themes in the Gathas, and allusions to conflict are really all they provide. However, Zoroastrian tradition from the following centuries (c.1000-500 BCE), also preserved in the Avesta, consistently refers to the militant role of Vistaspa. He was one of the few "good" Kavis in Zarathustra's time, the first king to convert to Zoroastrianism. Several of the Yashts, hymns dedicated to various divinities and heroes, reference Vistaspa's victories and preparations for battle against the other Kavis in defense of the faith (Yt 5, Yt 9, Yt 19). Yasht 24 is even a hymn celebrating Vistaspa himself, addressing the same concepts in its opening lines:

Give him strength and victory! Give him welfare in cattle and bread!' thus said Zarathustra to the young king Vîstâspa! Give him a great number of male children, praisers and chiefs in assemblies, who smite and are not smitten, who smite at one stroke their enemies, who smite at one stroke their foes, ever in joy and ready to help.

Later Zoroastrian literature, the Greater and Lesser Bundahishn, refer to Vistaspa's campaigns against the Daiva worshippers as the 'War of Religion.' Other parts of the Avesta, primarily other Yashts and the Vendidad, describe Zoroastrian ritual preparation for war including 'proper' arms and armor for divinely sanctioned war.

In terms of sources and information, we move forward, or at least adjacent, to the Achaemenid Persian Empire (553-329 BCE). This is the era that earned the ancient Persians a reputation for religious tolerance. While it's true that they didn't force Zoroastrianism on others and were happy to support other religious institutions across their empire, that doesn't mean that Zoroastrian beliefs weren't used to support their wars among their fellow Zoroastrians. Darius the Great's Behistun Inscription is a particularly good example, with Darius repeatedly crediting the Zoroastrian God, Ahura Mazda, for his victories over various rebels. I've discussed other aspects of Achaemenid religious conflict on this sub before. The most direct example, and my favorite, comes from Xerxes' Daiva Inscription.

Zoroastrianism is much less visible in the surviving sources for the Hellenistic and Parthian periods, but if we skip ahead to the Sassanid Persian Empire (224-651 CE), then Zoroastrianism becomes much more visible than any other period in history. This is due in no small part to the House of Sasan's apparent origins as priests in Istakr and an explicit alliance between the Zoroastrian clergy and the Sassanid kings from the very beginning. The Letter of Tansar, though likely edited in the 5th-7th Centuries, is a 3rd Century appeal from Tansar, the Herbedan Herbed (literally Judge of Judges, meaning the chief of the religious scholars), to a king in Tabaristan. Tansar argued strenuously that Ardashir I's wars and right to rule were divinely sanctioned.

A generation later, another priest, Kartir, became one of the most prominent militant clerics of the Sassanid period. Starting his career as another Herbed, he was eventually promoted to Mobedan Mobed (Priest of Priests, aka High Priest), and his stance is most clearly stated in his inscription on Kaba-ye Zartosht at Naqsh-e Rostam. In that text, Kartir not only justifies the wars of the various kings he served under in religious terms, but celebrates the persecution of other religions:

And in kingdom after kingdom and place after place throughout the whole empire the services of Ahura Mazda and the Yazads became preeminent, and great dignity came to the Mazdayasnian religion and the mobeds in the empire, and the Yazads and water and fire and small cattle in the empire attained great satisfaction, while Ahriman and the devs were punished and rebuked, and the teachings of Ahriman and the devs departed from the empire and were abandoned. And Jews, Sramans (Buddhists), Brahmins (Hindus), Nasoreans (Orthodox Christians), (Gnostic) Christians, Maktak (Baptisers or Mandaeans), and Zandiks (Manichaeans) in the empire were smitten, and destruction of idols and scattering of the stores of the devs and god-seats and nests was abandoned. And in kingdom after kingdom and place after place many divine services in magnificence and many Warharan fires were established, and many mobeds became happy and prosperous, and many fires and mobeds were imperially installed. And in documents and imperial rescripts and records, under Varahran, King of Kings, son of Varahran, which were made, in was recorded, "Kartir, Varahran's Soul-Savior, Ahura Mazda's Mobedan Mobed."

The Sassanid Empire fluctuated in how tolerant of other religions it was depending on the preferences of the kings, domestic politics, foreign affairs, and the strength of the clergy at any given time. Some kings were generally willing to let other religions flourish, while others authorized pogroms against various minority groups within the empire.

Militarily, this manifested most often in the struggle for power over Armenia, which had been a hub of Zoroastrian, or Zoroastrian-esque, religion since the Achaemenid period. This conflict is evident from Kartir's time as well. An inscription from Shapur I's on the Kaba-ye Zartosht lists Armenians as Eran, while Kartir's inscription from just a few decades later lists them as Aneran. While Middle Persian Eran can accurately be translated as an ethnonym like 'Iranian' or 'Aryan' (and Aneran as Non-Iranian/Non-Aryan), it also had an explicitly religious component. To be Eran also meant to be Zoroastrian, and by changing the category, Kartir was stating that the Armenians' heterodox version of Zoroastrianism was unacceptable.

This compounded with the Armenian king, Tiridates III, converting to Christianity in 301 and the Christianization of the Roman Empire over the following century. With the advent of official Christianization in Rome, there were two empires preaching their own religious supremacy and Armenia was caught in the middle, both geographically and religiously.

The religious component is especially evident in the late 5th-early 6th Century. Christian opposition to Sassanid-enforced Zoroastrianism was a driving motivator of several Armenian rebellions against Peroz I, but his brother, Balash, was more amenable to Christianity upon coming to power. Balash allowed Armenia to secede and Christians to operate unimpeded. This put him at odds with some of his more staunchly religious, and expansionist nobles, and civil war ensued. Balash had so few allies within the empire that he actually had to raise an army of Armenian allies to retake power, which only made him more unpopular and led to his eventual assassination.

The general ebb-and-flow of official Zoroastrian hostility toward other faiths continued throughout the rest of Sassanid history, though there are fewer surviving sources to provide context in the last centuries of their rule. Schisms within Christianity also lessened the explicit religious tone of later conflicts with the Romans, as the Sassanid kings used support for whichever Christian faction was out of power in Constantinople at the time to their advantage.