First time in Islamic history, Hazrat Ali was choosen by the muslims will. After the death of the third caliph, the whole muslim ummah got frightened. The tweleve years of misrule shocked the Muslims. So they deciede to hire such a leader. Who will think of their interests not his own? Everybody pointed in the favour of Hazrat Ali. They think that only Hazrat Ali retains the capability to lead us. in this way, Hazrat Ali becomes Caliph.