Complex sentence: Palmyrene trade reached its apex during the second century, aided by two factors; the first was a trade route built by Palmyrenes, and protected by garrisons at major locations, including a garrison in Dura-Europos manned in 117 AD.
Split into simpler sentences: Palmyrene trade reached its apex during the second century, aided by two factors. The first was a trade route built by Palmyrenes, and protected by garrisons at major locations, including a garrison in Dura-Europos manned in 117 AD.