Aga Khan (Persian: آغا خان; also transliterated as Aqa Khan and Agha Khan) is said by author Farhad Daftary to be the hereditary title of the Imam of the Nizari Ismaili community (although in a legal proceeding, the Aga Khan III noted that Aga Khan is not a title, but instead a sort of alias or "pet name" that was given to the Aga Khan I when he was a young man). The Nizari Ismaili community is a denomination of Ismailism within Shia Islam and consists of approximately 5-15 million adherents (under 10% of the world's Shia Muslim population). The Nizari Ismailis recognize the Imamat of the descendants of Ismail ibn Jafar, eldest son of Imam Ja'far al-Sadiq. On the other hand Twelver Shi`ism, the largest denomination of Shia Islam, recognizes the Imamat of the descendants of Ismail's younger brother Musa al-Kazim. The Fatimid Caliphate was ruled by seven leaders, after which a dispute regarding leadership developed between two brothers – Nizar and Mustali – ultimately resulting in the defeat of Nizar and his followers ("Nizari" Ismailis), many of whom left Egypt for the region of Iran.
In 1887, the colonial rulers of India, the Secretary of State for India acting through the Viceroy of India, formally recognized the title Aga Khan. During the latter stages of the First Anglo-Afghan War (in 1841 and 1842), Aga Khan I and his cavalry officers provided assistance to General Nott in Kandahar Province and also to General England in his advance from Sindh to join Nott. For these services, and others which Aga Khan I rendered to Sir Charles Napier in his conquest of Sindh in 1843–44, the Aga Khan received a pension from the British Government of India. He was awarded his the status of "Prince" by the British government's representatives in India and became the only religious or community leader in British India granted a personal gun salute; all other salute dynasties were either rulers of Princely States, or Political Pensioners holding ancestral princely titles in states abolished by the British. When Hassan Ali Shah, the first Aga Khan, came to Sindh from Afghanistan, he and his army were welcomed by Mir Nasir Khan of Sindh.