It appears that Ali did not take the oath of allegiance to Abu Bakr immediately. He however took the oath after some time. In his oration at the funeral of Abu Bakr, Ali spoke in glowing terms about Abu Bakr both as man and a caliph. When Abu Bakr nominated Umar as his successor Ali did not feel happy at the nomination. He, however, took the oath of allegiance to Umar. Ali even married his daughter to Umar, and the relationship between Ali and Umar was throughout cordial. When Othman was elected as the caliph Ali took the oath of allegiance to him. When Ali offered allegiance to his predecessors the implication is that he acknowledged their caliphate, and waived his own right to the caliphate even if he had any claim. When Ali himself acknowledged these Caliphs, it is not clear how does it lie in the mouth of anyone to say that these Caliphs were usurpers.