Intensive irrigation agriculture of the lower Tigris and Euphrates and of tributaries such as the Diyala and the Karun formed the empire's main resource base.
To the south and the.west lay the Arabian deserts, inhabited largely by Arab tribesmen who occasionally acknowledged the overlordship of the Sasanian Emperors.
Until 602, the desert frontier of greater Iran had been guarded by the Lakhmid kings of Al-Hira, who were themselves Arabs but ruled a settled buffer state.
In that year, Shahanshah Khosrow II Aparviz rashly abolished the Lakhmid Kingdom and laid the frontier open to nomad incursions.
The aristocratic and administrative upper class was mostly Persian, and the urban centers were a diverse mix of Iranians and Aramaic-speakers, while the rural population consisted mostly of the latter.
The Monophysites were regarded with more suspicion and were occasionally persecuted, but both groups maintained an ecclesiastical hierarchy, and the Nestorians had an important intellectual centre at Nisibis.
The area around the ancient city of Babylon now had a large population of Jews, both descendants of the exiles of Old Testament times and local converts.
Later, the tide began to turn, and in 627-628, the Byzantines, under the leadership of the Heraclius, invaded Khvārvarān province and sacked the imperial capital at Tyspawn (Ctesiphon).
By the end of the following year (638), the Muslims had conquered almost all of the Western Iranian provinces (modern Iraq), and the last Sasanian Emperor, Yazdegerd III, had fled to central and then northern Iran, where he was killed in 651.