the conditions under which the frontier army was operating. Inexpensive German imports increased the supplies available to the military, while at the same time relieving the local populations of the full burden of attempting to produce those supplies themselves. German cavalry and other auxiliary units soon were joining the Roman army, and the pressure of recruiting levies among the frontier populations relaxed. The Germans soon had money that they were willing to exchange for products manufactured in the frontier districts. This provided the local populations with cash for taxes, with enough left over for investment in the local economy. Perhaps the Romans could have established and maintained their frontier without the assistance of the German trade, but in fact they did not. At each of the critical steps in the process of establishing the frontier, the German trade provided the vital margin of success.
In another sense, however, the growth of the German trade eventually frustrated Roman plans. The early emperors had seen the frontier as a line separating them from their barbarian neighbors. It never was. The presence of the Roman army of the frontier in fact helped to create a great, although turbulent, German trading system that was soon integrated into the Roman money economy and the markets and products of the frontier districts. The Roman border provinces and free Germany formed a single economic entity. In later centuries, as the productive capacities of the Western Empire waned and its frontier defenses were stripped, the Romans' trading partners--Ostrogoths, Visigoths, Vandals, Sueves, Franks, Alans, Burgundians, and other border tribes--would cross the frontier in attempts to restore the situation. A reunification of the old trading region eventually came about, but not until the expansion of Charlemagne's empire in the early ninth century.