An analytical distinction, first made more than two thousand years ago by Thucydides, continues to shape most intellectual consideration of war. The Peloponnesian conflict destroyed the Athens of Pericles even more completely than the First World War did Europe, and Thucydides found its cause not in the immediate train of events that led up to it—the crisis in Epidamnus, the Corinthian and Corycrean interventions, the Megarian Decree, and so forth—but in something much deeper, namely “the growth of Athenian power, which presented an object of fear to the Spartans and forced them to go to war.”

Most modern historians have followed Thucydides’ lead and accepted that wars have “underlying” as well as “immediate” causes, and that, of the two, the underlying causes are the more important. Greatly strengthened by the rise of sociology, this approach has contributed to the eclipse, in historical writing, of the study of diplomacy and of battles and military strategy.

Most modern historians have followed Thucydides’ lead and accepted that wars have “underlying” as well as “immediate” causes.

An important exception to the pattern, however, is the work of Donald Kagan. His justly celebrated four-volume study of the Peloponnesian War argues that the underlying causes adduced by Thucydides were in fact not present: Athenian power was not growing during the period leading up to the war; “the imperial appetite of the Athenians was not insatiable but, under the leadership of Pericles, was satisfied fully; … the Spartans as a state were not