Umayyad Caliphate
The Umayyads and the Hashimites were bitter rivals. The rivalry came from the initial opposition of Abu Sufyan ibn Harb, the grandson of Umayya, to Muhammad and to Islam. He tried to get rid of the new religion by waging a series of battles. But eventually he accepted Islam, as did his son (the future caliph Muawiyah I), and the two provided much-needed political and diplomatic skills for the management of the quickly expanding Islamic empire.