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Ismāʿīl ibn Ja‘far al-Mubārak (Arabic: إسماعيل بن جعفر; c. Born:Shawwal 100 AH/719 AD) was the eldest son of Imam Ja'far al-Sadiq, and he was the full-brother of Abdullah al-Aftah. Their mother, Fatima bint al-Hussain'l-Athram bin al-Hasan bin Ali, was the first wife of Ja'far al-Sadiq. Following Ja'far's death, the Shia community split between the element that would become the Twelver Shia, and those who believed the Imamate passed through to Ismail's son; the Ismaili branch of Shia Islam is accordingly named for Ismail. According to both the Nizari and Mustaali Shia sects, he is the sixth Imam. He was buried in Jannat al-Baqi.
The Ismāʿīlī-Ithna’asheri Division
A major crisis arose among the Shia after the death of Imam Ja’far al-Sadiq, who had five sons. Abdullah and Ismail were the eldest sons by his first wife Fatima, a granddaughter of "Hazrat Hasan ibn Ali". Imam al-Sadiq did not take a second wife as long as Fatima was alive. Thus, there was a considerable gap in the ages of Abdullah and Ismail, on the one hand; Musa al-Kazim, Ishaq and Muhammad were Imam Sadiq’s three other sons from Hamida of Sudanese origins, on the other. Ismail was probably the second son of Imam al-Sadiq. He was about 25 years older than Musa, his younger half-brother. The exact date and circumstances of Ismail’s death also remain obscure. Some Ismaili authors relate that he survived his father, but a large number of Shia sources report that he pre-deceased his father by five years.
According to the majority of the available sources, Imam Ja’far al-Sadiq had indeed designated Ismail by nass (divine decree) as his successor in Imamate. After the death of Imam Ja’far al-Sadiq a great confusion arose amongst his sons as each of his surviving sons claimed the Imamat but could not produce sufficient credentials, and so their followers melted away in a short period except for two candidates; Ismail and Musa. Ultimately the majority of the Shia favoured Musa al-Kazim, a younger son of Imam Ja’far al-Sadiq and half-brother of Ismail. The Twelver Shia, however, believe that Imam Ja’far al-Sadiq did not decree a nass in favour of Ismail, and his only nass was in favour of his son Musa al-Kazim.
Thus after the death of Imam Ja’far al-Sadiq (148AH/765 CE) a major split came about among the Shia community. One section of Shia recognised the Imamat of Musa al-Kazim (d. 182 AH/798 CE) as their Imam and followed the future Imams from his progeny. The line of Musa al-Kazim continued until the twelfth Imam Muhammad al-Mahdi who is said to have disappeared at a very young age in Samara (Iraq) in the year 260 AH/873 CE, and is still the awaited Imam by the great majority of Shias at the present time. This group of Shia, the followers of the twelve Imams, are known as Ithna’asheri or Twelvers. 
Imam Ismail and the Commencement of the Dawr-al-Satar (Ismaili Period of Concealment)
The second group of the Shia recognized Ismail as the legitimate Imam, who they believe had not pre-deceased his father Imam Ja’far al-Sadiq. With regard to the Shia Twelver claims that Imam Ismail had pre-deceased Musa al-Kazim, the Ismailis believe that Imam Ja’far al-Sadiq observed taqiyya (dissimulation)  and gave a chance to his real successor to go underground so that their enemy, the Abbasids, did not pursue Ismail, and that his Imamat and his activities went un-noticed. Thus Musa al-Kazim who was believed to be poisoned by the Abbasid Caliph Harun was in fact a veil (hijab) for his elder brother Ismail. Ismailis believe that Musa al-Kazim gave his own life as a sacrifice for the sake of his brother Ismail, the true Imam. 
The Ismailis further argued that the Imam being masum (infallible) could not make an error of Judgement and therefore the first nass (designation) of Imam Ja’far al-Sadiq was the correct one. Thus, this group accepted Hazrat Ismail as their Imam and are known as Ismailia or Ismāʿīlīs.
Strong Abbasid persecution had put the entire Shia movement on guard and had indeed driven the general run of the Shias, particularly the Ismāʿīlīs, underground. The Abbasid authorities considered the Ismaili Imams as their arch political rivals and enemies, but in spite of their organised intelligence service, could not catch up with the Ismaili Imams, and their whereabouts remained unknown to them for a very long time. The secrecy maintained by the Ismaili dawa, as well as the Imams’ location away from Baghdad, the Abbasid capital, helped the Ismaili underground movement considerably.
Maqam al Imam is one of the two mausoleums in the city of Salamiyya that are of special significance to the Ismāʿīlīs living there. The locals mentioned that this shrine holds the tombs of Imam Taqi Muhammad and Rabi Abdullah. Some also believe that Imam Ismāʿīlī is buried here. The building is built on top of tunnels. During the Dawr al-Satr (period of concealment), the Imams resided in Salamiyya and its environs to avoid arrest by the Abbasids. On occasions when the soldiers threatened, the Imams were whisked up via underground tunnels. There is a famous story of Ismāʿīlī fidais claiming to be the Imam and risking arrest in order to confuse and delay the soldiers, giving the Imam time to flee.
During this period, the Imams settled in Salamiyya, near Hamma in Syria, but their identity and whereabouts were known only to a few completely trusted disciples. The four Imams who had succeeded Imam Ismail – Muhammad ibn Ismail, Wafi Ahmad, Taqi Muhammad, and Rabi Abdullah – while maintaining anonymity, were engaged in the creation of a remarkable network of mission centers equipped with a very well-developed and organized religious philosophy which came to be known as dawa . The term dawa - although used by some non-Ismāʿīlī circles – was the skillful organization and a highly elaborate and sophisticated network of communication within the community and unique to the Ismāʿīlīs. . The Ismāʿīlī faith retained its vitality in this period, during which the identities of the Imams remained protected, living as they were in hazardous circumstances. This period has been described as dawr- al-satar (period of concealment).
The Ismaili Imams carried out their mission from the secret hideouts from the year 148AH/765 CE until the time Imam al-Mahdi (the 5th generation of Imam Ismail) emerged as the rightful Imam in Sijilmasa, and established the Fatimid Caliphate in North Africa in the year 297 AH/909 CE. 
The Ismaili Caliphate, first in North Africa and then in Egypt, lasted for 285 years with their purpose-built capital al-Qahira (Cairo) with a majestic Mosque Jami’ al-Azhar and the very first University of the world, long before Oxford or Cambridge were planned. The Imam of the present age of the Ismaili community is Shah Karim al-Hussaini, His Highness the Aga Khan 4th, who is the 49th Imam in the direct unbroken chain from the progeny of Imam Hussain and Imam Ismail.
Some of his ancestors, important descendents and the tree of the Ismāʿīlī Shia Islam
Isma'il ibn Jafar, Abu Mohammad Ismail
Imam of Nizari Ismailism
|Rank||Sixth Ismāʿīlī Imām|
|Titles||az-Azbab-i-Itlaq (Absolute lord), Al-Wafi|
|Mother||Fatima bint al-Hussain'l-Athram bin al-Hasan bin Ali|
|Children||Muhammad ibn Ismail (successor), Ali, Fatima|
- John Norman Hollister (1979). The Shi'a of India. Oriental Books Reprint Corp. : [exclusively distributed by Munshiram Manoharla]. Retrieved 8 February 2011.
- Imam Ismail's house in Syria
- Complete History of Leadership
- History of the Ismaili Imams Tarikh-e Imamat
- ISMAIL BIN JAFAR SADIK (148-158/765-775), 6TH IMAM | Ismaili.NET - Heritage F.I.E.L.D.
- Ismaili History
Isma'il ibn Jafar
of the Ahl al-Bayt
Clan of the QuraishBorn: 103 AH ≈ 721 A.D. Died: 138 AH ≈ 755 A.D.
|Shia Islam titles|
|6th Imam of Ismailism
predeceased his father
Muhammad ibn Ismail