During his co-regency, he helped his father with the conquest and destruction of the city of Hatra, whose fall was facilitated, according to Islamic tradition, by the actions of his future wife al-Nadirah.

[1] Shapur later took advantage of the political turmoil within the Roman Empire by undertaking a second expedition against it in 252/3–256, sacking the cities of Antioch and Dura-Europos.

He did not seem interested in permanently occupying the Roman provinces, choosing instead to resort to plundering and pillaging, gaining vast amounts of riches.

Shapur had new Zoroastrian fire temples constructed, incorporated new elements into the faith from Greek and Indian sources, and conducted an extensive program of rebuilding and refounding of cities.

According to 5th-century BCE Greek historian Herodotus, the Median king Astyages wanted to have his grandson Cyrus killed because he believed that he would one day overthrow him.

[4] According to the modern historian Bonner, the story of Shapur's birth and uprising "may conceal a marriage between Ardashir and an Arsacid princess or perhaps merely a noble lady connected with the Parthian aristocracy.

[9] The background of the family is obscure; although based in Pars (also known as Persis), they were not native to the area, and were seemingly originally from the east.

[13] Since the end of the 3rd or the beginning of the 2nd century BCE, Pars was ruled by local dynasts subject to the Hellenistic Seleucid Empire.

[17] The Iranologist Touraj Daryaee argues that the reign of Vologases V was "the turning point in Parthian history, in that the dynasty lost much of its prestige.

[20] The Sasanian family had meanwhile quickly risen to prominence in Pars, and had now under Ardashir begun to conquer the neighbouring regions and more far territories, such as Kirman.

[19] Shapur, as portrayed in the Sasanian rock reliefs, took part in his father's war with the Arsacids, including the Battle of Hormozdgan.

[1] The battle was fought on 28 April 224, with Artabanus IV being defeated and killed, marking the end of the Arsacid era and the start of 427 years of Sasanian rule.

[23] Ardashir celebrated his victory by having two rock reliefs sculptured at the Sasanian royal city of Ardashir-Khwarrah (present-day Firuzabad) in Pars.

[25][22] The second relief, conceivably intended to portray the aftermath of the battle, displays the triumphant Ardashir I being given the badge of kingship over a fire shrine from the Zoroastrian supreme god Ahura Mazda, while Shapur and two other princes are watching from behind.

[25][24] Ardashir considered Shapur "the gentlest, wisest, bravest and ablest of all his children", and nominated him as his successor in a council amongst the magnates.

Al-Tabari alleges he rebuilt the ancient city of Zrang in Sakastan (the land of the Sakas, Sistan), but the only early Sasanian period founding of a new settlement in the East which is certain is the building by Shapur I of Nishapur—"Beautiful (city built) by Shapur"—in Dihistan (former Parthia, apparently lost by the Parthians to the Kushans).

[26] Soon after the death of his father in 241 CE, Shapur felt the need to cut short the campaign they had started in Roman Syria, and reassert Sasanian authority in the East, perhaps because the Kushan and Saka kings were lax in abiding to their tributary status.

[27] Shapur could now proudly proclaim that his empire stretched all the way to Peshawar, and his relief in Rag-i-Bibi in present-day Afghanistan confirms this claim.

[28] Shapur I claims in his Naqsh-e Rostam inscription possession of the territory of the Kushans (Kūšān šahr) as far as "Purushapura" (Peshawar), suggesting he controlled Bactria and areas as far as the Hindu-Kush or even south of it: [29][full citation needed] I, the Mazda-worshipping lord, Shapur, king of kings of Iran and An-Iran… (I) am the Master of the Domain of Iran (Ērānšahr) and possess the territory of Persis, Parthian… Hindestan, the Domain of the Kushan up to the limits of Paškabur and up to Kash, Sughd, and Chachestan.He seems to have garrisoned the Eastern territories with POW's from his previous campaign against the Medes of the Mountains.

Agathias claims Bahram II (274–293 CE) later campaigned in the land of the Sakas and appointed his brother Hormizd as its king.

When Hormizd revolted, the Panegyrici Latini list his forces as the Sacci (Sakas), the Rufii (Cusii/Kushans) and the Geli (Gelans / Gilaks, the inhabitants of Gilan).

[30] Ardashir I had, towards the end of his reign, renewed the war against the Roman Empire, and Shapur I had conquered the Mesopotamian fortresses Nisibis and Carrhae and had advanced into Syria.

"We have penetrated as far as Nisibis, and shall even get to Ctesiphon," the young emperor Gordian III, who had joined his father-in-law Timesitheus, exultantly wrote to the Senate.

[1] Philip immediately issued coins proclaiming that he had made peace with the Persians (pax fundata cum Persis).

[36] The victory over Valerian is presented in a mural at Naqsh-e Rustam, where Shapur is represented on horseback wearing royal armour and a crown.

However, just as with the above-mentioned Gilaks deported to the East by Shapur, the Persian treatment of prisoners of war was unpleasant but honourable, drafting the captured Romans and their Emperor into their army and deporting them to a remote place, Bishapur in Khuzistan, where they were settled as a garrison and built a weir with bridge for Shapur.

[38] However, the Persian forces were later defeated by the Roman officer Balista and the lord of Palmyra Septimius Odaenathus, who captured the royal harem.

[42] Under Shapur, the Iranian military experienced a resurgence after a rather long decline in the 2nd and 3rd centuries, which gave the Romans the opportunity to undertake expeditions into the Near East and Mesopotamia during the end of the Parthian Empire.

[44] Although Iranian society was greatly militarised and its elite designated themselves as a "warrior nobility" (arteshtaran), it still had a significantly smaller population, was more impoverished, and was a less centralised state compared to the Roman Empire.

From his titles we learn that Shapur I claimed sovereignty over the whole earth, although in reality his domain extended little farther than that of Ardashir I. Shapur I built the great town Gundishapur near the old Achaemenid capital Susa, and increased the fertility of the district with a dam and irrigation system—built by Roman prisoners—that redirected part of the Karun River.