Valens and the Goths - Gibbon's Decline and Fall of the Roman Empire Chapter 26 Part 3

Since the time of the first emperor. Augustus, there had been a rough equilibrium between the empire and the German tribes.  The empire had been too big for the Germans to defeat, but too overextended to threaten their independence.  But with the Huns now about to wipe out the Germans the position had changed.  The equilibrium was wrecked.