Issus (Ĭs´əs), ancient town of SE Asia Minor, now in Turkey, 5 mi (8 km) NW of Dörtyol. Located near the head of a gulf (the modern Gulf of Iskenderun), Issus was on a narrow strip of land backed by high mountains. Nearby, in 333 BC, Alexander defeated the forces of Darius III of Persia. In AD 194, Septimius Severus conquered Pescennius Niger. In 622 the Byzantine emperor Heraclius won at Issus the first of a series of battles in which the west regained territory formerly lost to the Persians.