Ammianus Marcellinus Res Gestae 31.7.3

[3] These troops, who had not yet learned the power of unbridled madness combined with desperation, drove the enemy beyond the precipitous crags of Mount Haemus and forced them into the steep defiles, in order that shut up in deserted and solitary places, and finding nowhere an outlet, they might be worn out by long continued hunger; they themselves in the meantime would await the coming of the general Frigeridus, who was on his way with the Pannonian and the transalpine auxiliaries, since Gratian, at Valens’ request, had directed him to take the field and bear aid to those who were harassed to the point of utter destruction.