 To stop the Byzantine harassment from the sea during the Arab–Byzantine wars, in 649 Muawiyah set up a navy, manned by Monophysitise Christians, Copts and Jacobite Syrian Christians sailors and Muslim troops, which defeated the Byzantine navy at the Battle of the Masts in 655, opening up the Mediterranean to Muslim ships.Early Muslim armies stayed in encampments away from cities because Umar feared that they may get attracted to wealth and luxury, moving away from the worship of God, accumulating wealth and establishing dynasties.Staying in these encampments away from the cities also ensured that there was no stress on the local populations which could remain autonomous.