The Prophet Mohammad died on 8 June 632, and 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 began a war of conquest. He launched campaigns against the Sassanid Empire and the Eastern Roman Empire (Byzantine Empire) and thus set in motion a historical trajectory that in just a few short decades would lead to one of the largest empires in history.