Following the death of his grandfather the Aga Khan III, Prince Karim, at the age of 20, became the 49th Imam of the Nizari Ismailis and Aga Khan IV, bypassing his father, Prince Aly Khan, and his uncle, Prince Sadruddin Aga Khan, who were in direct line to succession. In his will, the Aga Khan III explained the rationale for choosing his eldest grandson as his successor (which marked the second time in the history of the Nizari Ismaili chain of Imamat that a grandson of the preceding Imam – instead of one of the sons of the preceding Imam – was made the next Imam):
