Uthman, the third Caliph was killed by the Muslims who were not happy with his nepotism. Now, the circumstances did not give him any chance to ensure the succession of a man of his choice. Muslims were, for the first time, really free to select or elect a Caliph of their choice; they flocked to the door of Ali Ibin Abe Taleb(as).

But during the 25 years which had passed since the death of The Holy Prophet nature and outlook of the Muslims had changed to such an extent that many prominent people found Ali's administration (which was hundred per cent based on absolute justice and equality, just like the goverment of The Holy Prophet) unbearable: they could not think of themselves being treated as equal to a non-Arab Muslim. So first Talha, Zubair and Aesha revolted; then Moawiya stood against Ali.

After the martyrdom of Ali(as) Imam Hassan wanted to continue the war with Moawiya. But most of his officers were, meanwhile, bribed by Moawiya; and many were the commanders who when sent ahead to intercept Moawiya, changed the side and went over to the enemy. In this situation, Imam Hassan(as) had to accept the offer of Moawiya to enter into treaty.

Now, after this treaty, the Sunnis said that military power is a valid way of acquiring constitutional Caliphate. Thus, the four constitutional ways of Caliphate carne into being.