I begin this issue with alluding to the era of Imam Sajjad (as). In my opinion, the entire struggles, from the era of Imam Sajjad (as), that is, the year 61, until 260 hijra, can be divided into three stages.
The First Stage, 61-135 hijra (H.) -- The struggle in this stage gradually intensifies, deepens, expands and reaches its peak until 135 H. when Saffah passes away and is succeeded by Mansur Abbasi. When Mansur comes to power, some difficulties are created and as a result the progress of the struggle is to a great extent retarded. Such developments are natural in a political struggle. We have observed similar developments in our own struggle.
The Second Stage, 135-202 (or 203) hijra -- This year marks the martyrdom of Imam Ridha (as). During this stage the intensity of the struggles is higher than that in 61 H.; they are more widespread and deeper, but at the same time they face new difficulties. They gradually spread and step by step get closer to the victory until the year of the martyrdom of the Eighth Imam (as). At this stage the struggles are once again stopped.
After Mamun had gone to Baghdad and began his rule in 204 H., one of the most difficult phases on the life of the infallible Imams began. The new phase is the era of the difficulties for the infallible Imams. Although Shiism spread more than ever during this period, in my opinion the difficulties the Imams faced were also more than ever during those days.
During this period the Imams launched their struggle to achieve their goal in the long run, that is, the Imams did not struggle to achieve their goals before the Minor Occultation of Imam Mahdi (May Allah Hasten His Reappearance), rather they prepared the ground for the ensuing period. This period, which began in 204, continued until 260 H. when Imam Askari (as) was martyred and the Minor Occultation of Imam Mahdi (as) began. Each of these three periods has certain characteristics, which I am going to touch briefly.