Complex sentence: According to the book "Hagarism", the Arab conquests and the formation of the caliphate were a peninsular Arab movement inspired by Jewish messianism, which, in alliance with Jews, attempted to reclaim the Promised Land from the Byzantine Empire.
Split into simpler sentences: According to the book "Hagarism", the Arab conquests and the formation of the caliphate were a peninsular Arab movement inspired by Jewish messianism. In alliance with the Jews, the Arabs attempted to reclaim the Promised Land from the Byzantine Empire.