A note of importance is the difference in the meanings of Imam and Caliph. The Imams refer exclusively to Imam Ali bin Abi Talib (as) and his sons Imam Hasan and Imam Hussein (as) and the nine sons descended from Imam Hussein (as). They were chosen by Allah (SWT), the Almighty and were not selected by the people. Whereas the Caliphs were elected to their offices in a political role. Unfortunately, Islam does not divide politics from other parts of life including religion and since the elected ones were not always qualified to interpret the religious aspects of their rule, many deviations have occurred and continue to occur in the Muslim community.
At the time of the death of the Holy Prophet Muhammad (saw), Imam Ali's (as) position became one of controversy and self-imposed isolation. He was selected by Allah (SWT) and nominated by the Holy Prophet Muhammad (saw) at a place called Ghadir Khum to take up the reins of leadership after the Holy Prophet Muhammad (saw). More then 100,000 Muslims witnessed this selection and nomination and gave their allegiance to him at that time and place. Still, it was nearly 25 years before he could take his rightful position of Caliphate. He ruled for less than 5 years before he met martyrdom at the hands of a paid assassin. At 63 years of age his earthly light was extinguished and his heavenly light shone once again.
His singular distinction is that he, Ali bin Abi Talib (as) was the Imam and fourth Caliph of the Muslims. His descendants were Imams, guiding the Muslims in religious matters, but none were able to truly take the political position of Caliph during their lifetimes. When it looked as though they might become such by popular choice, they were brutally assassinated except for the last of them, Imam Muhammad Mahdi (as), on whom we await.