Full quote,
"War was rarely launched against the Romans and they always waged war at a time and in a manner of their own choosing on those whom they regarded as most convenient."
The other reason is because the Romans thought war as "the only art" and devoted "mind entire and all their thoughts to its perfection." He said,
"Never has a nation made preparations for war with so much prudence and conducted it with so much audacity."
I read this in Paul Rahe, "Empires: Ancient and Modern" in Wilson Quarterly.
Perhaps we should consider the military history of the Republic too? Considering only the period from Augustus onwards leaves out something like half of Rome's history, and the period of its greatest expansion.