Twenty years after the Vietnam war, Robert McNamara, United States secretary of defence during the Kennedy and Johnson presidencies, explained why ratios between Viet Cong/North Vietnamese Army and US/Army of the Republic of Vietnam body counts were used to measure military effectiveness in Vietnam: "Things you can count, you ought to count. Loss of life is one." Thucydides, the Athenian general-turned-historian, would have appreciated McNamara's blunt pragmatism, but we can only imagine with what understated ironies the Greek would have treated decisions in Washington and events in Southeast Asia between 1954 and 1975.

In Kennedy's Wars, Lawrence Freedman gives us a Thucydidean take on how the Kennedy White House handled cold-war trouble spots. He invites us to make other calculations. On November 15 1961, Kennedy was adding up the numbers in Vietnam and resisting persistent proposals for aggressive military action from General Maxwell Taylor and Walt Rostow, then deputy special assistant to the president for national security affairs. JFK was wary of "intervening in an area 10,000 miles away against 16,000 guerrillas with a native army of 200,000". Freedman views the president's own service and near-death experience in the second world war as strong factors in how he weighed advice about the deployment of US military power. JFK was "alert to the pain of battle. He had a frontliner's suspicion of headquarters' bombast and a sceptical view of the judgement of senior commanders."

McNamara lent his support to proposals for military solutions in Vietnam and Laos. He confesses in retrospect that he too, at this juncture, was probing into "the complexity of the situation and the uncertainties of our ability to deal with it by military means". Neither McNamara nor Rostow shared JFK's empathy with the soldiers doing the fighting. McNamara efficiently fine-tuned the figures, while Rostow ignored Vietnamese cultural and historical factors and kept the emphasis on the need to use air and naval power against the external factors in Viet Cong insurgency: North Vietnam and even Red China.

Vietnam veteran and ancient historian Lawrence Tritle, in studying the massacre at My Lai in 1968, would have us think about the soldiers on the ground, while we calculate the tragic numbers of "McNamara's war": "The first official reports of the day's action at My Lai - the 128 Viet Cong guerrillas killed, the three weapons captured." Later investigations, connected with the court martial of Lt William Calley, fixed the body count at 504 old men and women, mothers, small children and infants. No further weapons were reported.

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Tritle equates the atrocity at My Lai with Athenian actions at Melos in 416-5BC. There, Athenian soldiers put to death all adult males, enslaved the women and children and gave over the territory to 500 Athenian settlers. Tritle sees Melos and My Lai as clear examples of actions taken by soldiers suffering from battle-induced post traumatic stress disorder (PTSD). The comparison is natural and euphonic. Melos and My Lai are the most infamous atrocities perpetrated by the "good guys" in the Peloponnesian and Vietnam wars. But the Melos side of the equation is problematical.

Tritle compares ancient Greek accounts of battle and its effects, notably in the tragedies of Aeschylus and Sophocles, the histories of Thucydides and Xenophon, and works of art, to accounts of the Vietnam war in histories, novels, short stories, memoirs, poems, film, and public memorials. Tritle keeps our focus on those who fought. He takes up the thesis of psychiatrist Jonathan Shay's influential Achilles in Vietnam . Shay argues that ÁñÁ«ÊÓƵric-style warfare caused damage to the psychological well-being of combatants identical to the PTSD symptoms of the Vietnam veterans under his treatment. Tritle extends Shay's thesis to soldiers who fought in the wars of the historical Greek city states and to others who fought in Vietnam. This is important because Shay was criticised by Vietnam veteran-author-poet William Ehrhart for drawing universal conclusions about the effects of combat from a very small sample. The background of soldiers, personal and cultural, surely affects their susceptibility to what Shay describes as the "ruin of good character" in combat. It is, therefore, pertinent to interpreting what Athenian soldiers did at Melos.

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Tritle's approach yields many good results. He argues persuasively that Aeschylus's sympathetic treatment of the defeated enemy in The Persians is consistent with the "survivor's perspective" he acquired from fighting at Marathon. We see similar sensitivities in the writings of other veterans: Tim O'Brien, Wilfred Owen, Siegfried Sassoon, William Broyles Jr and Erich Maria Remarque. Tritle acknowledges the difficulty in drawing explanatory parallels between ancient and modern cultures with unique values, ethics and morals and unique developmental histories. The ancient historian Peter Green has recently discussed the false retrojection of modern ideas of morality into ancient Athenian thinking about the same event, the destruction of Melos, singled out by Tritle and Thucydides. Yet, comparative historical analysis can shed light on both subject periods. Thucydides wrote his history with this belief: that it would serve as a handbook of test cases of how humans behave during a war between what we now call superpowers.

In 1947, at the outset of the cold war, the US secretary of state, George Marshall, believed Thucydides: "I doubt seriously whether a man can think with full wisdom and with deep convictions regarding certain of the basic international issues today who has not at least reviewed in his mind the period of the Peloponnesian war." Marshall assumes a working knowledge of Thucydides, and his advice was well taken - for a time. For example, the Foreign Service Journal for August 1952 contains "A message from Thucydides".

As we read Freedman's meticulous analysis of the formulation of point-of-conflict foreign policy within the Kennedy administration, we have good cause to lament that between January 1961 and November 1963, no one consulted Thucydides. But we, at least, can heed Marshall's advice as we review our recent history. A prior reading of Thucydides's history gives us a tragic sense of d¨¦ja vu when we then follow Freedman's reconstruction of interlinked decisions and events relating to Berlin, Cuba and Laos that inexorably created the Vietnam quagmire.

Francis Ford Coppola in his film Apocalypse Now interpreted the Vietnam war in Conradian terms: apocalyptic acts of brutality revealing the thinness of the veneer we call civilisation. JFK himself saw Vietnam presciently as Sassoon might have viewed it: an alcoholic's uncontrollable descent into more than a lost decade. Two apt visions. Name your poison.

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One radical difference between Freedman's intellectual history of Kennedy-era foreign-policy decision-making and Thucydides and Tritle on ancient Greece is obviously the quality and quantity of pertinent documentation. Freedman sifts carefully through presidential and national security archives; congressional and state department reports; transcripts of internal White House recordings; memoirs; histories, biographies and autobiographies of major and minor advisers; and their writings on policy theory, contemporary journalism, including pieces by columnists whom JFK intentionally cultivated in order to better assess and direct public opinion; other scholarly assessments of the Kennedy presidency; and secondary sources that draw on documentary material from the other side, ie Moscow. Thus, he is able to construct convincing fly-on-the-wall arguments against any false impression of presidential militancy.

For the Peloponnesian war, we have mainly Thucydides, supplemented by fragmentary inscriptional evidence, later synoptic histories, contemporary comic and tragic plays and archaeological reconstruction. But Thucydides is something the world has seen but once: a top-level general in a cold war evolving into a world war, privy to high-level superpower decision-making, who in the course of the war goes into exile and studies subsequent events from the other side. Imagine General Patton in Moscow as the Russian forces drive toward Berlin.

Still, Thucydides's test-case approach has hidden snares. He treats major events as paradigmatic: Corcyra of the escalating intensity of civil war when opposing sides each have a superpower big brother - or think they do; Plataea of how superpowers themselves become subservient to the demands of strategic allies; and Mytilene of vacillation in central democratic decision-taking. He gives only the information necessary to construe each paradigm.

Was Melos a My Lai-style rampage by soldiers and local commanders suffering from PTSD? Let us calculate. The democratically determined good result of Athenian second thoughts about Mytilene was to destroy the city walls, kill 1,000 men (a sizeable percentage of the overall population) and confiscate all Mytilenean land. Let us add up other numbers. A board of five senior Spartan judges at Plataea, after weighing arguments and counter-arguments, decided to kill the entire surviving adult male population (more than 200 men and 25 Athenians), sell the women and children into slavery, raze the city and expropriate its territory. These actions were taken in the same annus mirabilis (4BC) as the revolution at Corcyra, and they were taken by Greeks against Greeks who had been their allies.

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When Athenian commanders 11 years later debated higher virtues and the pragmatics of power with selected Melian commissioners, both sides knew what to expect if Melos was taken by siege. We are told by Thucydides that after the Melian debate, most of the Athenian force returned to Athens. Reinforcements were then sent out again with Demeas in command. Demeas pressed the siege to completion and then followed standard operating procedure: kill the men, enslave the women and children and confiscate the land. This was not a PTSD-induced local command decision carried out by soldiers who had "gone off". Demeas and his troops did what had to be done. Thucydides knew commanders like Demeas on both sides. He had been one of them. He would tell us that Demeas was no William Calley and Melos no My Lai.

Tom Palaima is professor of classics, University of Texas at Austin, United States, where he teaches seminars on ancient and modern war and violence.

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