Following the death of Muhammad in 632, there was a vigorous push by the Arab Muslims to conquer Arab tribes of the East such as the mostly Christian Ghassanids. The Byzantine-Muslim Wars were a series of wars between the Arab Muslims Caliphates and the Eastern Orthodox Byzantine Empire. These started during the initial Muslim conquests under the Rashidun and Umayyad caliphs and continued in the form of an enduring border tussle until the beginning of the Crusades.