The collapse came fast. In 633 CE, Rashidun Arab armies invaded both Byzantine and Sasanian territory simultaneously. By 637, the Sasanian capital Ctesiphon had fallen, and with it all of Iraq and large swaths of Syria. Khuzestan, one of the richest provinces in the Sasanian realm, became the next target. Hormuzan, an Iranian aristocrat who governed the region, launched guerrilla raids from his base at Hormizd-Ardashir -- the city now known as Ahvaz. The Sasanian king Yazdegerd III, ruling in exile, supported these raids and believed the lost territories could be recovered. It was not an unreasonable hope. The Arab armies were stretched across thousands of kilometers. But each time Hormuzan sued for peace, he broke the terms. Each time he broke the terms, he lost more ground.

Shushtar's defenses were ancient and layered. The canals that ringed the city bore the names of the rulers who had built them. One was called Ardashiragan, after Ardashir I, founder of the Sasanian dynasty in the 3rd century CE. Another was called Shamiram, named for the legendary Assyrian queen Semiramis, linking the waterworks to traditions far older than Persian rule. A third bore the name Darayagan, after the Achaemenid king Darius I, who ruled in the 5th century BCE. These were not just irrigation channels. They were moats with a thousand years of engineering behind them, fed by the Karun River and deep enough to stop cavalry. When Hormuzan arrived after a defeat at Ram-Hormizd, nine hundred of his soldiers had been killed and six hundred captured. The survivors who reached Shushtar with him placed their trust in water.

The siege ground forward without breakthrough until a defector changed everything. According to the historian al-Tabari, an Iranian named Sina approached the Arab commander and offered to reveal a way into the city in exchange for his life. His instructions were direct: attack through the outlet of the water channels. Al-Nu'man took a small force and entered Shushtar through the waterways that had been designed to keep invaders out. Al-Bara' ibn Malik led the infiltrating units who fought their way to the city gates and opened them from inside. A different account in the Khuzestan Chronicle describes defectors from the region offering the same information in exchange for a share of the plunder. The historian al-Baladhuri adds another detail: during the siege, a group of elite Iranian cavalry known as the Asawira defected to the Arab side, choosing to preserve their status and wealth under new rulers rather than die for a dynasty that was already finished.

Hormuzan retreated to the citadel when the city walls were breached, but his resistance could not last. He surrendered and was sent as a captive to Medina, the capital of the Rashidun Caliphate. There, faced with a choice between conversion to Islam and execution, he converted. For a time he lived in Medina as a political figure of some significance -- a defeated Sasanian lord navigating the world of his conquerors. But suspicion followed him. When the caliph Umar was assassinated in 644 CE, Hormuzan was accused of involvement in the plot by Umar's son Ubayd Allah ibn Umar, who killed him extrajudicially without trial. Meanwhile, the fall of Shushtar accelerated the conquest of Iran. By 651, the last Sasanian king Yazdegerd III was dead, assassinated on the orders of his own governor at Merv. With his death, most of Iran was under Arab control. The canals of Shushtar still flowed, but the empire they had been built to protect was gone.