Muawiyah I had been appointed the governor of Syria by Umar in 639 to stop Byzantine harassment from the sea during the Arab-Byzantine Wars. He succeeded his elder brother Yazid ibn Abi Sufyan, who died in a plague, along with Abu Ubaidah ibn al-Jarrah, the governor before him, and 25,000 other people. Now under Uthman's rule in 649, Muawiyah was allowed to set up a navy, manned by Monophysitic Christians, Copts, and Jacobite Syrian Christian sailors and Muslim troops, which defeated the Byzantine navy at the Battle of the Masts in 655, opening up the Mediterranean.