The Islamic prophet Muhammad died on 8 June 632, Abu Bakr succeeded him as first Caliph. Abu Bakr's Caliphate lasted for 27 months, during which he crushed the rebellion of the Arab tribes throughout Arabia in the successful campaign against Apostasy and restore the authority of Madinah over Arabia. Once the rebellions had been put down, Abu Bakr realized that the Sassanid Empire and the Byzantine Empire both threatened the borders of the nascent Muslim state and that passiveness would only lead to invasion. He therefore launched campaigns against the Sassanid Empire and the Byzantine Empire, setting in motion a historical trajectory that in just a few decades would lead to one of the largest empires in history.