clipped from: www.livescience.com The Persian Sasanians ruled Mesopotamia from 224 AD. They thrived for centuries, but eventually became distracted by fighting the Romans and amongst themselves. Ultimately, an unlikely outside force would topple them. The Arabs had been tribesmen, unorganized and militarily primitive. The new religion of Islam, founded by Muhammad, united the tribes. In 634, the Arab campaign against the Sassanians began. 18,000 Arab tribesmen, led by General Khalid ibn al Walid ("The Sword of Islam") reached the Euphrates delta and began battling the Iranians (Persians), who were rallied by their hero, Rustam. A decisive battle occurred at Al-Qidisiyah, a village south of Baghdad. Though outnumbered six to one, the Arabs defeated the Iranians, exhausted by many battles against the Byzantines. Rustam was killed. The Arabs shortly captured the Sassanid capital at Ctesiphon, ending their dynasty and introducing Islam to the region.