Imam Hasan Askari (as) and Holy Prophet Muhammad's tradition about Twelve Successors

In the early days of his Imamate of our 11th Imam, Imam Hasan Askari
[who succeeded his father, Imam Ali Naqi in 254 AH / 868 CE at the age
of 22], Al-Mu'tazz was the Abbasid Caliph. When the latter was
deposed, he was succeeded by al-Muhtadi, son of al-Wathiq. After his
brief reign of only eleven months and one week, Al-Mu'tamid eldest
surviving son of al-Mutawakkil came to the throne. During their
regimes, Imam Hasan Askari (as) did not enjoy peace at all. Although
the Abbasid dynasty was involved in constant complications and
disorders, each and every Caliph thought it necessary to keep Imam
Hasan Askari (as) imprisoned.
One of the Holy Prophet Muhammad's traditions ran that the Holy
Prophet Muhammad (saw) would be succeeded by twelve successors from
his progeny ( Ahlul Bayt), the last of whom would be Imam al-Mahdi
(as).
The Book on Government (Kitab Al-Imara)
Sahih Muslim :: Book 20 : Hadith 4480
It has been narrated on the authority of Jabir b. Samura who said: I
heard the Messenger of Allah (may peace be upon him) say: Islam will
continue to be triumphant until there have been twelve Caliphs. Then
the Holy Prophet (may peace be upon him) said something which I could
not understand. I asked my father: What did he say? He said: He has
said that all of them (twelve Caliphs) will be from the Quraish.
Judgments (Ahkaam)
Sahih Bukhari :: Book 9 :: Volume 89 :: Hadith 329
Narrated Jabir bin Samura: I heard Prophet Muhammad saying, "There
will be twelve Muslim rulers (who will rule all the Islamic world)."
He then said a sentence which I did not hear. My father said, "All of
them (those rulers) will be from Quraish."
The Abbasids knew well that the true successors of the Holy Prophet
Muhammad (saw) were these very Imams. Imam Hasan Askari (as) being the
eleventh, his son would surely be the 12th or the last. They, tried to
put an end to his life in such a way, that it would ensure there would
be nobody to succeed him. But these Abbasid Caliphs had forgotten how
Firaun (Pharaoh) had tried to stop the reign of Prophet Musa / Moses
(pbuh)by killing all the baby boys born to the Israelites, yet he
himself was bringing up the one he wanted to kill.
As the simple confinement inflicted on his father, Imam Ali Naqi (as)
was considered inadequate for Imam Hasan Askari (as), so he was
imprisoned, away from his family. No doubt the revolutionary intervals
between two regimes gave him brief periods of freedom. Yet as soon as
the new Caliph came to the throne, he followed his predecessor's
policy and imprisoned Imam Hasan Askari (as) again. Imam Hasan
Askari's (as) brief life (28 years) therefore was mostly spent inside
dungeon cells.
Moral:Allah (SWT) says in the Noble Qur'an, "And they planned, and
Allah planned, and Allah is the best of planners." (Noble Qur'an 3:54)
i.e. Whatever Allah (SWT) has decided no-one can change.