and was one of the principal centres of a developed polytheistic
worship in the peninsula. Mecca's commercial and religious signifi-
cance gave its leading tribe, the Quraysh, advantages over other,
principally nomadic tribes of western Arabia, which seemed threat-
ened by the new movement.' In 622 Mul).ammad, to avoid a planned
assassination, was able to take the opportunity of a request by some
of the inhabitants of Medina to move there with his followers, to act
as a mediator in the current inter-tribal feuding that was dividing the
town. This exodus, known as the Hijra was subsequently taken as the
founding date for the Islamic calendar. Once established in Medina,
Mul}.ammad was able to impose his religious reforms on some of the
local tribes and to expel others, including some that practised Judaism.
He was further able, thanks to Medina's strategic position on the
caravan routes to the north of Mecca, to impose an economic block-
ade on the latter, cutting its trade to the Byzantine provinces. Failing
in successive attempts to break his hold by force, the Meccans were
eventually obliged to capitulate in 630, accepting Mul}.ammad's reli-
gious reforms. In the last two years of his life the Prophet went on to
make many of the other tribes of Arabia receive his teachings and
political suzerainty. This included the Yemen, which had been con-
quered by the Persians around the time of Mul,lammad's birth.
A reaction, called the ridda, set in with his death in 632. Many
tribes apostatised, while a crisis of leadership developed in Mecca.
The latter was resolved when, setting aside 'Ali, Mul}.ammad's nearest
male relative and son-in-law, a small group of the Prophet's closest
associates selected Abu Bakr, traditionally his first disciple, to be cal-
iph ('successor') or leader of the Muslim community. During his
brief tenure of this new office (632-634) Abu Bakr and his advisors
put an end to the Ridda or apostasy amongst the tribes, and instead
launched them into simultaneous attacks on the territories of the two
great empires to the north, the Byzantine and Sassanian Persian. This
had dramatic consequences; during the rule of the second caliph,
'Vmar (634-644), another former companion of Mul).ammad, armies
of both empires were defeated in successive engagements. Syria (635),
Palestine (636) and Egypt (641/2) were lost by the Byzantines, Meso-
potamia (636/7), including their capital city of Ctesiphon, and west-
ern Persia (642) by the Sassanians. In 636 Jerusalem fell to the Arabs,
and 'Vmar entered the city, riding on a donkey.B
The initial impetus of conquest was halted after the murder of the
third caliph, 'Vthman (644-656), who had over-endowed his own
