after muhammad died in 632, a new head of the state, called caliph, was named: abu bakr. he was the first of the four caliphs whose caliphate was called that of the rashidun, which means 'rightly guided.' after dealing with the ridda wars, wars against rebellious tribes who rebelled after muhammad's death, he started to campaign against the byzantines. abu bakr himself did not live to see the results of his initiative after suffering a natural death in 634. but his successors sure did.