Here is the seventh part of the Introduction from the book titled 250 Year Old Human by Ayatullah Khamenei.
Introduction [Part 7]
If the Aa’ima (as) only carried out these two duties and nothing else, it would have been enough for us to say that the life of the Aa’ima (as) from the start to the end was political. Those places where they would say commentary [of the Quran] and those places where they would be expressing the understanding of Islam, in reality, they were carrying out a political duty. In places where they were mentioning the specifics of an Imam, again, that too was a political act that they were carrying out. Meaning if the education of the Aa’ima (as) consisted of just these two features, these two practices and these two topics that we have mentioned and just finished at that, still [that would be enough to say] that the life of the Aa’ima (as) was a political one, however, it was not just restricted to this. Apart from all this, the Aa’ima (as), at least from the time of Imam Hasan Mujtaba (as) and onwards, had started an underground political and revolutionary movement with the objective of capturing the Government. That which I am stating is unfamiliar, this is the main point of this case that, unfortunately, in the books that have been written in the lifetimes of the Aa’ima (as) about the life of Imam Jafar al-Sadiq (as), in the life of Imam Musa ibn Jafar (as) and in the lives of a lot of the other Aa’ima (as), this point has not been introduced.
This that the Aa’ima (as) carried out a structured and broad political movement, even with all the witnesses, has remained unsaid and unmentioned, and this is one of the principle things which has been an obstacle for understanding the life of the Aa’ima (as). The truth is that the Aa’ima started their political duties and there were witnesses [to see this too].
So as a summary the brothers and sisters should know that just by taking on the trusteeship of Imamat, all of the Aa’ima (as) one of the things they started was a political struggle and a political endeavor to capture the Government. This endeavor was just like the endeavor which anyone might carry out for the formation of a system, the Aa’ima (as) also carried this out.