When the Roman and Parthian Empires first collided in the 1st century BC, it appeared that Parthia had the potential to push its frontier to the Aegean and the Mediterranean. However, the Romans repulsed the great invasion of Syria and Anatolia by Pacorus and Labienus, and were gradually able to take advantage of the weaknesses of the Parthian military system, which, according to George Rawlinson, was adapted for national defense but ill-suited for conquest. The Romans, on the other hand, were continually modifying and evolving their "grand strategy" from Trajan's time onwards, and were by the time of Pacorus able to take the offensive against the Parthians. Like the Sasanians in the late 3rd and 4th centuries, the Parthians generally avoided any sustained defense of Mesopotamia against the Romans. However, the Iranian plateau never fell, as the Roman expeditions had always exhausted their offensive impetus by the time they reached lower Mesopotamia, and their extended line of communications through territory not sufficiently pacified exposed them to revolts and counterattacks.