Charles Oman,
The Byzantine Empire
“ Never before had either power dealt its neighbour such fearful blows as in this last struggle: in previous wars the contest had been waged around border fortresses, and the prize had been the conquest of some small slice of marchland. But Chosroës and Heraclius had struck deadly blows at the heart of each other's empire, and harried the inmost provinces up to the gates of each other's capitals. The Persian had turned the wild hordes of the Avars loose on Thrace, and the Roman had guided the yet wilder Chazars up to the walls of Ctesiphon. ”
