Res Gestae

This at least I would declare without any hesitation: the Gauls, since they have been harassed by long continued troubles and grievous misfortunes, cannot voluntarily or under compulsion send recruits to foreign and distant countries, for fear that, if they lose all their young manhood, downcast as they are by the memory of their past afflictions, even so they may perish from despair at what may befall hereafter.

Furthermore, it will not be expedient to draw from here auxiliaries to be opposed to the Parthian nations, since the onsets of the barbarians are not yet checked and (if you will permit me to speak the truth) these provinces which have been vexed with constant calamities need aid themselves from without, and valiant aid too.