Syria was one of the greatest empires back then. It was large and the military then was very powerful. Syrians had a tough life so they would be trained very well. In 539 B.C.E Syria became part of the Persian Empire so that made it even stronger. Persia was the most powerful empire until Alexander came to demolish it. In 331 B.C.E, Alexander the great gathered his troops and surprised Persia by uniting all of Rome against them. As he destroyed many places on his way, he came to Syria. After Persia concurred Syria they became weak. So when Alexander came, he had no worries.