If the incoming barbarians were sufficiently numerous eventually to overcome that section of the Roman army that could be pointed in their direction, why did not the Empire simply raise more troops? The answer to this question lies in the limitations of its economy. Late Roman agriculture was, if anything, booming in the Fourth Century, but there was no obvious means of quickly or substantially raising output. In many provinces, the economy was operating at maximal levels of output. It is unlikely, therefore, that there was much extra slack left by the year 400 to fund still larger armies after the major increases in tax extracted a century earlier to fund the new armies required on the Persian front. The Empire’s tax-take was also slightly limited by its bureaucratic capacity and the willingness of local elites to pay, but there is little sign that it was having much trouble with taxpayers before the 440s, when Aetius had to rein in tax privileges after the loss of North Africa. The most significant limitation on taxation would appear to be the buoyant but plateaued economy.

From Peter Heather’s The Fall of the Roman Empire: A New History, London: Pan Books, 2006 (first published in 2005), pp. 447-448.