The first of the four major caliphates established after the death of Muhammad was the Rashidun (632–61), which was characterized by a period of rapid military expansion in the Middle East and North Africa, followed by a five-year period of civil war between the Caliph Ali, Muhammad’s son-in-law, and Muawiyah, the governor of Syria, who had consolidated his local power base and built up a powerful army and navy. The Umayyads were a clan of the Quraish tribe, which dominated Mecca before the rise of Islam. The victorious Muawiyah became the founding caliph of the Umayyad Dynasty in 661, with its capital at Damascus. Muawiyah and his successosrs waged unceasing war on the Christian Byzantine Empire and oversaw military expansion in North Africa and Central Asia. Despite disputes over hereditary accession, the Caliphate continued to consolidate power, establishing Arabic as its official language, introducing a Muslim coinage, and continuing its warfare against the Byzantines. In 712 the Umayyads sailed into the Persian Gulf and conquered the Sindh and Punjab regions of the Indus River, but further eastern expansion was halted by the Battle of Asksu (717) when Tang dynasty Chinese defeated Umayyad invaders. The long reign of Caliph Hisham (724–43) marked the high point of military expansion; from bases in North Africa a series of raids on the Visigothic Kingdom paved the way for the permanent occupation of Iberia by Umayyad invaders. Following a series of revolts in the 740s, caused by financial crises, feuds and factionalism, the Umayyad dynasty fell, succeeded by the Abbasids in 750; in 756 a new Umayyad dynasty was established in Spain, based in Córdoba, under the rule of Abd ar-Rahman.