This more favourable condition was first of all due to the fact that many scientific and philosophical books were translated from Greek, Syriac and other languages into Arabic, and the people eagerly studied the intellectual and rational sciences. 

Morever, Al-Mamoun, the Abbasid calipha from 198/813 to 218/833, had Mu'tazilite leanings and since his religious views he favoured intelletual demonstration, he thus was more inclined togive complete freedom to the disscussion and propagation of different religious views. 

The Shia theologians and scholars took full advantage of this new freedom and did their utmost to further scholarly activities and propagate Shia Teachings. 

Also Al-Mamoun, following the demands of the political forces at the time, had made the 8th Shia Imam(as) his successor, as is recounted in most standard or if not all histories. 

As a result, the Descendants of The Holy Prophet(phuh&hf) and their friends were to a certain extent free from pressures from the government and enjoyed some degree of liberty. 

Yet before long the cutting edge of the sword once again turned towards the Shia and the forgotten ways of the past came upon them once again. 

This was particularly true in the case of Al-Mutawakkill (233/847 to 247/861) who held a special enmity towards Ali Ibin Abe Taleb(as) and his Shia. By his order the holy and sacred Tomb of the 3rd Holy Imam(as) in Karbala was completly demolished and flooded over, by diverting water from the nearby river .