The Abbasids were able to beat the Umayids in 750, and the first Abbasid Caliph was Abu al-Abbas al-Saffah. The Abbasids moved the heart of the Muslim Empire eastward with power coming from Iraq and Iran. Al-Saffah was caliph from 750-754. The Abbasids then turned on the Shiis who had previousely supported them believing the Abbasids were the rightful rulers because they were descendents of Mohammed and would rule according to the Quran. However the Abbasids had mislead the Shiis and ruled much like the Umayyids whom they had disposed of for ruling like a monarchy. The Abbasids knew that the only way the Muslim Empire could be run was to have one centralized authority making all of the decisions of the state.
Abu Jafar al-Mansur ruled from 745-775 during his time of rule, al-Mansur killed any Shii leader that opposed his rule. He moved the Muslim capital from Mecca to Baghdad. By 750 Islam had spread from Arabia to the east as far as Afghanistan, Pakistan, parts of Turkey, all of North Africa, parts of Spain, and was stopped just short of entering Europe by the Byzantine Empire. The new capital was known as Madinat al-Salam, "The City of Peace. His son al-Mahdi succeeded his father in 775. Caliph Harun al-Rashid (786-809) ruled the Muslim Empire like a monarch. He changed the position of the caliph, the caliph would let viziers handle the affairs of the umma, and the caliph would distance himself more from the umma
The Abbasids were ruling like monarchs which was what they fought the Umayyids for doing; The Abbasids modeled their form of government on the Persian style of rule. The Caliph was put into a position of power where he ruled in luxury and conducted affairs of state with great ceremony. The Abbasids had brought stability and prosperity under their rule by ruling as a monarch and taking power away from the umma.
Until about the ninth century, the Abbasids rule was the supreme rule in the Middle East. But around the ninth century and even before then, there were parts of the Muslim Empire that were not under the control of the Abbasids but fell under the control of the Shiites, the Umayyids, or the Kharijites. These various factions vying for control left the Muslim Empire in a vulnerable position.
In 1095, Pope Urban II, would urge
Christians to retake the Holy Lands in the middle east. The
First Crusades began, with thousands of Christian soldiers pouring
into the Muslim Empire. Within fours years
the Christians would take Jerusalem. Over the following
centuries Muslims and Christians would take and retake lands from
one another.The Abbasids were weak from
infighting among themselves. They were however able to expel
the Europeans under the leadership of Salah al-Din or as he is
known by the Europeans as Saladin. Salah al-Din retook the
Holy Landsin 1187 but the Muslim Empire was still a weak
empire. The Muslims in the 13th century would face
a new enemy from the west coming out of central Asia, the
Mongols. The invasion of the Mongols would end the rule of
the Abbasids and the Muslim Empire would split even
further.