Abu Bakr's caliphate was short but important. An exemplary leader, he lived simply, assiduously fulfilled his religious obligations, and was accessible and sympathetic to his people. But he also stood firm when, in the wake of the Prophet's death, some tribes renounced Islam; in what was a major accomplishment, Abu Bakr swiftly disciplined them. Later, he consolidated the support of the tribes within the Arabian Peninsula and subsequently funnelled their energies against the powerful empires of the East: the Sassanians in Persia and the Byzantines in Syria, Palestine, and Egypt. In short, he demonstrated the viability of the Muslim state.