Complex sentence: Palmyra and Petra which are located on the fringes of the Syrian Desert, flourished mainly as power centers of trading they would be the source of supplies for the merchants caravans and policing the trade routes.
Split into simpler sentences: Palmyra and Petra which are located on the fringes of the Syrian Desert, flourished mainly as power centers of trading. They would be the source of supplies for the merchants caravans and policing the trade routes.