The last of my five foreign policy questions for the candidates that I posed prior to the Democratic debate was about the Thucydides Trap, a coinage of Graham Allison’s to describe the way rising powers and established hegemon’s frequently stumble into catastrophic war that benefits neither:

More than 2,400 years ago, the Athenian historian Thucydides offered a powerful insight: “It was the rise of Athens, and the fear that this inspired in Sparta, that made war inevitable.” Others identified an array of contributing causes of the Peloponnesian War. But Thucydides went to the heart of the matter, focusing on the inexorable, structural stress caused by a rapid shift in the balance of power between two rivals. Note that Thucydides identified two key drivers of this dynamic: the rising power’s growing entitlement, sense of its importance, and demand for greater say and sway, on the one hand, and the fear, insecurity, and determination to defend the status quo this engenders in the established power, on the other.

In the case about which he wrote in the fifth century B.C., Athens had emerged over a half century as a steeple of civilization, yielding advances in philosophy, history, drama, architecture, democracy, and naval prowess. This shocked Sparta, which for a century had been the leading land power on the Peloponnese peninsula. As Thucydides saw it, Athens’s position was understandable. As its clout grew, so too did its self-confidence, its consciousness of past injustices, its sensitivity to instances of disrespect, and its insistence that previous arrangements be revised to reflect new realities of power. It was also natural, Thucydides explained, that Sparta interpreted the Athenian posture as unreasonable, ungrateful, and threatening to the system it had established—and within which Athens had flourished.