Despite repeated assassination attempts by Shahryar's agents, Yazdegerd and his family found shelter in India. His son and grandson established a powerful state there, the Third Indo-Sassanid Empire, but continued to harbour ambitions to retake Iran. In 718 therefore Yazdegerd's grandson Narsieh, allying with disaffected elements among the Persian nobility and with the tacit support of Rome, launched an invasion into the eastern Iranian Plateau.
From 718 to 726 fighting raged across the region, causing much suffering, but eventually Narsieh emerged triumphant in the south and was crowned emperor in Ctesiphon. However, his rival, Shahryar's great-grandson Khusrau IV, was able to withdraw with the bulk of his forces into central Asia and continued to wage war. With the aid of a mixed Chinese-Turkish army sent by his ally, the Xuanzong Emperor of Tang, Khusrau struck south, conquering Gandhara and the Indus Valley and thereby cutting Narsieh's forces off from their heartland in India.
Narsieh died in 732 and was succeeded by his son Hormizd VII. Hormizd and Khusrau fought on for several more years in the Kushan and Indian provinces, until Khusrau was eventually assassinated on the orders of his generals and ministers, who were tired of war. Khusrau's son Ardashir immediately started to negotiate peace, and in 736 travelled to Ctesiphon to pay homage to Hormizd before retiring to his estates in Kerman.
Though peace had returned, the Sassanid state emerged badly weakened. Trade routes had been diverted away from Iranian lands, cities and farms lay in ruins everywhere, and Turkish nomads had taken the opportunity to take over and settle in large parts of Khwarezm and Khorasan. Hormizd and his successors would strive for years to restore Persian power, but the empire would only briefly return to its past glory before its final decline in the 9th century.