The red history of Shiaism of Imam Ali (AS) proved that whosoever accepted the Islam minus the family of Imam Ali (AS) and struggled on its way did reach no where and was finally rendered despicable and helpless and at last turned into the tool of the hands of imperialists of the history.

Al-Mansur conspires against Imam Jafar as Sadiq (AS),Since the cultural school of thought of Imam Jafar as Sadiq (AS) considered human beings effective in their fate and told them it is you who form a society.
It is you who make yourselves prosperous or unfortunate and miserable. Only you provide the grounds for your progress or your fall. Meaning thereby, it is you who can dismiss the rulers from the seat of power and do not accept cruelty and hand over the ruler ship to the men who are capable and sympathizers. The propagation and spread of this belief and idea, which was the same Quranic belief of the Muslims, displeased and annoyed al-Mansur the second Abbasid Caliph.
Al-Mansur the second Abbasid Caliph tried that he might arrest and stop the flow of this belief by propagation of negative values about morality and corruption of ideas and practices. Thus refrain the people from joining the school of thought of Imam Jafar as Sadiq (AS), which is the school of information, consciousness and movement.
al-Mansur the second Abbasid Caliph employed scholars for the implementation of his aims so that they may make the people believe in the religion of suppression and constraint. That is to say, they wanted to mould the belief of people and make them believe that whatever poverty, woe, and oppression and excesses are going on, all these are from God and human being do not have any interference to reshape it. So that as a result of it the opposition, revolt and uprising are stopped.
People may tolerate the excesses and cruelties of the tyrant rulers and bear the punishments and whips of their criminal masters. They, instead of objecting upon them, may express Commendation and thankfulness. Evidently and clearly Imam Jafar as Sadiq (AS) could not watch the propagation of such a wrong and dangerous belief, which was given to the people and taught in the name of Islamic culture and beliefs. Imam Jafar as Sadiq (AS) laid the foundation of the Islamic University to combat against this dangerous imperialistic belief.
In a short span of time, he trained four thousand students in the field of Islamic culture and beliefs and spread them among the people to get the effects of the propaganda of the agent scholars, neutralized and defused.
al-Mansur the second Abbasid Caliph’s Excesses over Imam Jafar as Sadiq (AS)Surprisingly, the Abbasides gathered the people around them on the pretext of taking the revenge of the blood of martyrs of Karbala and fight against the brutalities of Bani Omaiydes and remove and replace the Bani Omaiydes through the help of Iranians who were inclined to and loved the family of Imam Ali (AS) and also with the help of Abu Muslim Khorasani.
But instead of handing over the caliphate to the Imam of the age i.e., Imam Jafar as Sadiq (AS) they themselves overtook the government. From the year 132 Hijra the period of Omaiydes was discontinued. Two caliphs named ‘Saffah’ and ‘al-Mansur’ overtook power. The first one ruled for 10 years while the second for a period of 22 years. All the time they teased and annoyed the Imams and religious leaders continuously. Imam Jafar as Sadiq (AS) was under a severe stress and pressure during the age of al-Mansur. Even to the extent that sometimes the people were stopped from seeing him. So far so, that when a person named ‘Haroon’ wanted to see Imam Jafar as Sadiq (AS) and enquire the solution to a problem. He started thinking what to do to meet the Imam Jafar as Sadiq (AS). Suddenly, he happened to see a street hawker who was selling cucumbers. He came near him, purchased all the cucumbers, borrowed his old dress, put it on, and pretending to be a cucumber seller went to Imam Jafar as Sadiq’s (AS) house and questioned him about his problem, received the answer and returned.
al-Mansur arrested most of the friends of Imam Jafar as Sadiq (AS) and imprisoned them and quite a few times decided to martyr the Imam Jafar as Sadiq (AS) but each time by the miracle of Imam Jafar as Sadiq (AS) he would be defeated.
Stability of Imam Jafar as Sadiq (AS) al-Mansur who had been disappointed after using various means and ways, started deceiving public and tried to manifest and show himself as the real heir of Holy Prophet Muhammad (SAW) and one of the persons fit for the post of Islamic caliphate, on the pretext that he is from the lineage of Abbas (Holy Prophet Muhammad’s uncle) and the family of Holy Prophet Muhammad (SAW). But he knew fully well that he did not have the caliber and capability of this and that post only suited the sons of the Holy Prophet Muhammad (SAW) "the Imams (AS)."Imam Jafar as Sadiq (AS) severely and strongly combated against this public deceiving conduct and degraded and disgraced this family through his letters.
1. One day al-Mansur wrote to Imam Jafar as Sadiq (AS) "why do you not come to see me like the others?" Imam Jafar as Sadiq (AS) wrote in his response. "We do not possess any thing from the world so that we may be afraid of you for it (loss of it). You too do not possess anything for the justice day and the spiritualism so why must I come to see you?
2. The next day al-Mansur wrote to Imam Jafar as Sadiq (AS) "Come here to admonish me." Imam Jafar as Sadiq (AS) wrote replying him, "whosoever be from the people of the world (adorer of world) will not admonish you and whosoever will be the man of the justice day will not come near you."
3. One of the days Imam Jafar as Sadiq (AS) was present in the assembly of ‘al-Mansur’. By chance, a fly started teasing ‘al-Mansur’ and howsoever he tried to make it buzz off it would not and again sat upon his nose and head.
Al-Mansur much annoyed as he was, said to Imam Jafar as Sadiq (AS), "Why has God created the fly?”Imam Jafar as Sadiq (AS) at once replied, "For the reason of disgracing and degrading the powerful tyrants through them.
"‘al-Mansur’ became annoyed; he sat comfortably on his place and allowed the Imam Jafar as Sadiq (AS) to leave. Conflict of Imam Jafar as Sadiq (AS) with ruler of Medina After al-Mansur had martyred a group of the family of Bani Hashim.
He sent a man named "Sheba" as the ruler of Medina to keep the activities of Imam Jafar as Sadiq (AS) under observation. He came to the mosque on Friday and after the prayers was over. Seated himself on the tribune and started taking liberty and using foul language about Imam Ali (AS) and the family of the Holy Prophet Muhammad (SAW). People who knew the Prophet’s family form a close quarters and loved them got extremely annoyed but they could not dare utter a word. At this occasion Imam Jafar as Sadiq (AS) got up and said, "Oh man! We deserve all those merits and virtues you have mentioned and you and al-Mansur deserve those demerits and vices you have spoken about.
Then he turned his face towards the people and said, "Do you know who will be the most of the looser on the day of justice, tomorrow. That person who sells his doomsday for the world (material gains) of the others and such is this corrupt ruler.
Martyrdom of Imam Jafar as Sadiq (AS) Although al-Mansur did his best to bring the Imam Jafar as Sadiq (AS) close to himself or make him silent by threatening him but he failed. So he decided to martyr Imam Jafar as Sadiq (AS).Consequently, the Imam Jafar as Sadiq (AS) was poisoned and martyred on 25th of Shawwal, 148 Hijrah. His pure body was buried in the graveyard of Baqi in Medina.

Source: tebyan