Shahin is first mentioned in 602, after the outbreak of the Byzantine–Sasanian War of 602–628, where he commanded the forces invading Byzantine territory in the Transcaucasia, winning a battle against Domentziolus near Theodosiopolis in 607/8. Following the expulsion of Roman forces from that region, in 611 Shahin led an advance into Anatolia, capturing Caesarea. There, Phocas' son-in-law Priscus, started a year-long siege to trap them inside the city. However, Shahin's troops escaped Priscus' blockade and burned Caesarea, much to Heraclius' displeasure. In 613 the Roman offensive pressed on into Syria, but the combined Persian armies under Shahin and Shahrbaraz crushingly defeated Heraclius near Antioch. After this victory the Persians looted the city, slew the Patriarch of Antioch and deported many citizens. Roman forces lost again while attempting to defend the area just to the north of Antioch at the Cilician Gates, despite some initial success. The Persians then captured Tarsus and the Cilician plain. This defeat cut the Byzantine empire in half, severing Constantinople and Anatolia's land link to Syria, Palestine, Egypt, and the Exarchate of Carthage.