They took advantage of the situation and formed an...
They took advantage of the situation and formed an association with the slogan that “ Right to rule is for the family of the Holy Prophet.” They pledged in one of their meetings in Madina in which Muhammad Nafse Zakiya, the great grandson of Imam Hasan (as) was present, that when the Ummayads state topples, the Khilafat will be reverted to the Ahlul Bayt. It is clear that to rule the Islamic world was not the job of every Hashemite or Abbasid.
It was the right of those divinely appointed descendants of the Holy Prophet whom God had chosen to lead humanity. But these high-thinking souls never wished to take undue advantage of the situation with the aid of cunning tactics. In short, the Imams, the descendants of ‘Ali (as) never tried to acquire power through political trickery. But the Abbasids no doubt took the opportunity of the situation.
Availing themselves of the silence shown by the Imams and of the compassion people had for the Hashemites (the Abbasids too were Hashemites) they saw their chances to rise to power. But when they established themselves on the throne, they became the enemies of Ahlul Bayt, in the same degree or more than that which had been adopted by the heartless Ummayads. One of the prominent agents of this revolution was Abu Salma al-Khallal who was especially compassionate towards the Ahlul Bayt.
When he gained power in Iraq he wrote letters to Imam As-Sadiq inviting him to accept and share the royal power. In political struggles, such opportunities are considered golden, but the Imam who was an embodiment of selflessness and self respect, declined the offer and remained devoted to his duties of imparting knowledge.
The supporters of the Abbasid cause and the followers of Abu Muslim Khorasani then took the oath of allegiance at the hand of Abul Abbas as Saffah on 14th Rabiusani 132 Hijri (30.11.749AD) he was acknowledged as the ruler and caliph of the Muslim world. In the year 136 Hijri Saffah died and his brother Abu Ja’far Mansur succeeded to the throne.
It has already been indicated that the Abbasids took advantage of the popularity of the Ahlul Bayt and had made their slogan to stand and protect the rights of Ahlul Bayt. They gathered the public around them on this pretext. But when they came to power and destroyed the Ummayads, they turned against the Ahlul Bayt, particularly the descendants of Imam Hasan (as). Abu Salama who was known for his love of the Ahlul Bayt, was killed by Saffah.