It was in the reign of Yezid II that the propaganda in favor of the descendants of Abbas was to be actively disseminated over the East. The Abbassid emissaries appeared in Khorasan in the garb of innocent merchants and diligently canvassed for Mohamed, who was now the head of the Abbassid branch. The Abbassid emissaries worked wherever they went, with their lives in their hands if caught they risked to the barbarous punishments. Adherents were enrolled on all side and within a short time Persia was honeycombed with secret organization to encompass the downfall of the hated family of Banu Ommeya. The Banu Abbas were the descendants of Abbas, cousin of Prophet Mohamed (Pbuh) who died in 654 leaving four sons: Abdullah, Fadl, Obaidullah and Kaisan.
Abdullah, better known in history and tradition as Ibn Abbas was born in Mecca in 619, three years before the Higera. All four brothers were present at the Battle of Camel and at Siffin. Ibn Abbas, who was no less an accomplished soldier than a scholar, commanded the cavalry of Ali. He died of a broken heart at Tayef in 685 after the murder of Hussein. His son (who was named Ali after the great Caliph) followed the footsteps of his father in his zealous attachment to the children of Fatima. When Ali died, his son Mohamed succeeded him in the headship of the family. Mohamed was a man of great ability and unbounded ambition. He was the first man to plan a project for seizing the caliphate for himself. So he started a new doctrine to justify the claims of his family to the Imamate. His doctrine held the belief that on the murder of Hussein at Karbela, the spiritual headship of Islam was not transmitted to his surviving son Ali Zain-ul-Arabidin but to Mohamed al Hanafia. Mohamed al Hanafia was the son of the Caliph Ali by a lady of the Hanifa tribe, whom he married after the death of Fatima that upon Hanafia's death, his office descended upon his son Abu Hashim who had assigned it to Mohamed Ibn Ali Ibn Abdullah.
Before his death, Mohamed named his sons Ibrahim Abdullah Abu Al-Abbas (surnamed As-Saffah) and Abdullah Abu Gaafar (surnamed Al-Mansur) as his successors, one after the other. The propaganda started by him was conducted after his death with lifetime. The Abbassids prepared themselves to revolt in Khorasan and formed a strong army under the command of Abu Moslem al Khorassany who advanced westward and captured Iraq. He defeated Merwan II in the Battle of Zab in 750, which sealed the fate of the Ommeyade Dynasty. Merwan fled towards Mosul, but the city closed its gates upon him. He then hurried to Harran and tried to raise another army. The Abbassids were on his track. Finding no safety in Syria and Palestine, he fled to Egypt and was killed. Abu al Abbas Abdullah ascended the throne and founded the Abbassid Dynasty.

Abu Gaafar Al Mansur, 754-775 

With the rise of the Abbassids, the aspect of Western Asia altered. The seat of government was removed from Syria to Iraq. The Syrians lost the monopoly of influence and power they had hitherto possessed and the tide progress was diverted from the West to the East. But the unity of the Caliphate was gone forever. Although As-Saffah, Al-Mansur's brother was the first caliph of the Banu Abbas, Abu Gaafar must be regarded the real founder of the dynasty. He laid the foundations of the caliphate which maintained and enhanced the prestige of the throne in religious and political matters, and in later years became the chief source of its strength and of its influence. Mansur opened the series of those brilliant caliphs who devoted themselves to the building of new cities, the construction of roads, canals, fountains, the formation of charitable and educational institutions, the stimulation and protection of letters and the promotion of commerce and all arts of peace. Hardly had Al-Mansur been seated on the throne than Abdullah Ibn Ali (his uncle who was a governor of Syria under As-Saffah) rose in revolt. Mansur dispatched Abu Moslem to crush the rebellion. Consequently, Abdullah was defeated, captured and then killed. After Abu Moslem's victory over Abdullah, he desired to return to the government of Khorasan, of which he had practically made himself the king. His power in the province was unbounded, and had indeed become a source of danger to the Abbassids. So long as Abu Moslem lived, Mansur did not think himself secure on the throne. When Abu Moslem was murdered, Mansur felt that he was indeed the ruler. He also suppressed all disturbances and crushed his enemies. Then he removed to Baghdad which is said to have been the summer retreat of Kisra Anushirwan (the great Chosroes), and found a great part of the city. It was the capital of Islam, the eye of Iraq, the seat of empire, and the centre of beauty, culture and arts".
For about 500 years later, Baghdad continued to be the centre of Islamic civilization, progress and a very good centre for exchanging goods. The city was circular in shape, and surrounded by double walls. The palace stood in the centre. In the days of its prosperity, the population of Baghdad and its suburbs amounted to over two millions. Mansur's authority was now acknowledged over Western Asia and Africa, and although Spain was not subject to his temporal sway, the Khutba was read in his name even in that country, as he was the custodian of the Holy Cities. As the Kurds were beginning to give trouble, Mansur appointed Khalid Ibn Barmak his Chancellor of the Exchequer and Governor of Mesopotamia. Khalid by a mixture of firmness and justice, soon brought the province into order. Mansur was the first to cause dissensions between the Abbassids and the Alides, for before that they were united. Abu Gaafar's character was a mixture of good and evil. As a politician, a statesman and a ruler, he is almost unsurpassed. Mansur reigned about 22 years. He was a thin, tall man of fair complexion and exemplary in his conduct and life. He slept little and rose early for the morning prayers. He personally reviewed his troops and examined fortresses. The army was fitted throughout with imposed weapons and armor.

The Golden Age of Harun Ar-Rashid, 786-814 

The reign of Harun ar-Rashid is the most brilliant period of Arab rule in Asia. Harun deserves well the admiration of posterity as one of the great rulers of the world. He was a soldier by instinct and training: he repeatedly took the field himself and frequently traversed his dominions in every direction to repress lawlessness, personally inspected the frontiers and never spared himself trouble in carrying out the work of government. The perfect immunity from danger, with which traders, merchants, scholars and pilgrims traveled through the vast empire, testified the excellence and vigor of his administration. The mosques, colleges and schools, the hospitals, dispensaries, roads, bridges and canals with which he covered the countries under his rule speak of his great interest in the welfare of his people. The glory and fame of Rashid's administration are mostly due to the wisdom and ability of the men to whom he entrusted the government of the Empire for the first seven years of his reign. As previously mentioned, Khalid Ibn Barmak occupied a distinguished position under Mansur. His son Yahia was entrusted by Mahdi with the education of ar-Rashid. When he had been nominated successor to the caliphate, he was made his counselor and his vizier.
The moment ar-Rashid came to the throne, he appointed Yahia as the Vizier of the Empire and vested in him absolute power. Yahia's administration was wise, firm and benevolent, and the well-being of the people was made of primary importance. His sons Fadl, Gaafar, Musa and Mohamed were also able men and possessed of administrative capacity of the highest order. Owing to old age, Yahia resigned and Gaafar was entrusted with the office. For seventeen years, this remarkable and gifted family governed the empire of ar-Rashid with fidelity. Their sudden fall furnishes an instructive lesson in intrigue under despotism. Risings and rebellion were quelled up to the time of Rashid: Afrikia, instead of yielding any revenue, had been a constant drain on the resources of the Empire. Ibrahim Ibn Aghlab, was appointed Governor of Afrikia, and the office was made hereditary. Not only should he restore peace and order in the province, but also remit annually to Baghdad 40,000 dinars. In Asia, the government was conducted on settled lines. At the time of Rashid, the whole of Kabul and Sanhar was annexed to the empire, and the frontier extended as far as the Hindoo Kush. At the same time, Rashid separated the marshes of Asia Minor form the ordinary governorship, and under the name of Awasim, placed them under the control of a special military governor. Tarsus (in Cilicia) was repopulated and converted into a strong fortress.
As usual, the Kharijis rebelled several times in his reign, but their risings were suppressed without difficulty. The riotous conduct of the people of Mosul led Rashid to demolish the walls of their city as a punishment. But Rashid's wars with the Byzantines are the most interesting events of his reign. Breaking treaty concluded, their army was repulsed with great slaughter. Harun defeated Irene and Nicephorous in many places. A treaty was concluded by Nicephorous and the princes of his family, by which he agreed to pay an increased tribute besides a personal impost on himself and on each member of the house. Rashid enlarged the office founded by his grandfather Mansur for the translation of scientific work into Arabic and increased its staff. Among the eminent men who flourished during the whole or part of his reign, may be mentioned Asmai the grammarian, Shafei, Abdullah Ibn Idris, Isa Ibn Yunis, Ibrahim el Mousili the musician, and Gabriel Ibn Bakhtiashu the physician. Communications were opened in his reign with the West so well as with the East, and he was the first to receive at his court embassies form Faghfur (Emperor of China) and from Charlemagne.

The Increasing Authority of the Turks during the Abbassid Dynasty

The Abbassid Dynasty came into being when the Persians were spiteful to the Omm eyades who preferred the Arabs to them. The Abbassids depended on the Persians and entrusted them with the greatest offices in the empire such as viceroyalty, viziership and the command of the army. The Persians seized this opportunity and took hold of all the power in the empire; consequently feud and strike took place between the Arabs and the Persians. The supreme power was in the Persians' hands until Mutasim b'llah, El Mamum's brother, succeeded to the caliphate. He formed a standing army composed chiefly of Turkish Mamelukes form Central Asia and they were commanded by their officers and at the same time were completely separate from the Arab and Persian troops.
The number of these Turks multiplied until it reached two hundred and fifty thousand soldiers and one hundred and sixty thousand horses. Not only was Mutasim's aim to lessen the authority of the Persians, but also to force the Arab soldiers to leave their military posts and indulge in agriculture and so he stopped paying their soldiers. No sooner did the Turks get these new offices than they galloped recklessly through the streets of Baghdad knocking down everybody in their way. Mutasim, fearing the anger of the capital, moved with his favorite new army to a place called Samarra or Durra-man-Ra'a, 60 miles north-east of Baghdad. When Al Wasik came to the throne after his father Mutasim's death, he aggrandized the Turks at the expense of the Arabs and the Persians. He appointed Ashnas the Sultan or the lieutenant of the Empire and decorated him with jeweled girdle and swords entrusting him with the administration of the Empire.

The Decline of the Abbassid Dynasty.

The premature death of Al Wasik was an irreparable calamity. The glory of the Abbassids ended with his death. Or the next two hundred years of the history of the Abbassids presents a confused picture of caliphs coming to the throne without power and descending to the grave without remorse. They were puppets in the hands of the Turkish army. The good caliph, who tried to show personal independence in the government of the empire, was destined to be dismissed, imprisoned, murdered and even blinded. Such was the decline of the dynasty. The Disintegration of the Dynasty The viceroys seized the opportunity of the weakness of the central government at Baghdad and ruled their provinces independently although the religious supremacy was still held by the caliphs at Baghdad.
Among the causes which led to the downfall of the Abbassid Dynasty are:
1) The Ommeyades used to depend upon the Arabs and gave them all the high offices. This aroused the jealousy of the Persian Muslims, so they wanted to put an end to the Ommeyade Dynasty in the hope of restoring their ancient pomp and glory. The inhabitants of Khorasan and the Persians thought of supporting the Abbassids and popularizing their cause, and in the end they succeeded in wresting the caliphate from the Ommeyades. So the Abbassids based their power, at the beginning, upon the Persian support. The Persians did all they could to alienate the Arabs from the Caliphs on the pretext that the Arabs were aspiring to the caliphate, because of their hereditary rights.The Persians began ruling the empire as is seen by the power of the Barmakides and those who came after them. Fearing the result, Al Mutasim multiplied the number of the Turkish Mamelukes and used them as soldiers to defend himself against the Persians and Arabs combined. Neither the Turks nor the Persians were loyal to the Abbassids, but they had other political tendencies, the result of which brought about the weakening of the Abbassids. When the power of the Turks increased, they tried to take over the country: they imprisoned the caliphs and put out their eyes and deposed those they disliked and gave high positions to those they liked. So after sometime, each race began wishing for the return of the time of their happiness and independence.
2) Bad economic conditions prevailed: after the first caliphs had consolidated the foundations of the empire, their successors indulged in easy living and luxury, which resulted in large expenses and the increase of taxes. Consequently, the source of wealth deteriorated and its power of levying it weakened.
3) Among the causes of the downfall of the dynasty was the appearance of Alid states. The rivalry between the Alides and the Abbassids brought about war between them. In these wars, the Alides took parts of the empire which were ruled independently by them such as the creation of the Idrisid Dynasty at Maghrib and Fatimid Dynasty (which extended from the Atlantic Ocean to Hijaz and Yemen) and the establishment of the Būyid Dynasty which had the supremacy over Baghdad and the rest of the Abbassid Empire.
4) The increase in races and the appearance of creeds unknown during the reign of the Ommeyades led to many quarrels which ended in weakening the empire.