What do the works of the Greek poet Homer have to do with the nitty-gritty details of personnel policy in today's U.S. Army?

Plenty, says Jonathan Shay. In fact, so much that the former assistant professor at Harvard Medical School, who has written two well-received books examining Homer as a chronicler of military men in "The Iliad" and "The Odyssey," signed on this month as an adviser to the Army's personnel chief. Shay's task is far from literary. Rather, it is to help boost "cohesion" -- that is, the essential psychological glue that holds soldiers together -- in Army units.

It is a long way from ancient Troy to today's Pentagon. But Shay sees a direct line.

In his first book, "Achilles in Vietnam: Combat Trauma and the Undoing of Character," he used Homer's account of combat in the Trojan War to examine the Vietnam War, and especially how poor leadership increased the trauma of many U.S. soldiers in that conflict. Shay's sequel, "Odysseus in America: Combat Trauma and the Trials of Homecoming," reinterpreted Odysseus's troubled voyage back to Ithaca as a way of understanding the long and painful journey home of many combat veterans.

Through his work as counselor of Vietnam veterans, Shay has become a passionate advocate of the three things that he has concluded reduce the trauma of war on soldiers: keeping members of units together, giving them good leadership, and putting them through intense and realistic training.

"Cohesion, leadership and training -- each of these is a protective factor against psychological injury," he said. And together, "the synergism is enormous."

So, he said, he sees his one-year stint at the Pentagon as a work of "preventive psychiatry."

Signing on with the Army at age 62 may seem to be an odd career move for someone who is a veteran not of the armed forces but of three different Ivy League universities. Indeed, everything about Shay's background paints him as an unlikely candidate to advise the military: a beard-adorned, yoga-practicing resident of Newton, Mass., who describes himself as a "lifelong liberal Democrat, and proud of it."

But, he explains, 17 years of counseling Vietnam veterans at what is now the Department of Veterans Affairs transformed him from being a detached academic into a zealot for cohesion in U.S. military units.

"I am a physician with a fire in the belly for prevention of psychological injury in military service," Shay wrote in a summary of his work. "As such I am the missionary for the injured veterans whom I serve in the VA. They don't want other young kids to be wrecked the way they were wrecked." Indeed, to not abandon his patients, he is staying with the VA part time during his Pentagon tour.

His goal of cohesion is easily explained but harder to achieve, he said.

" 'Cohesion' is really about mutual trust," he said. "If you don't have mutual trust, you tend to burn up all your physical and emotional resources." For example, a soldier in the front lines who distrusts his comrades' ability to protect him from the enemy will not be able to sleep well. "If your gaze is directed inward -- 'Can I trust these guys?' -- then your cognitive resources are directed inward, when they should be directed outward, toward the enemy," Shay said.

The notion extends beyond small units such as squads and platoons. When subordinate commanders trust and understand their superiors, entire large divisions and corps become more militarily effective, he argued.

"Trust lubricates the friction of warfare," he said. "If every move in the chain of command has to be formally laid out, you are going to move slowly, and the enemy is going to move faster than you."

Another oddity of his move to the Pentagon is that it comes essentially after he and his allies in personnel policy have won much of the argument, especially on unit stabilization. After decades of transferring people every couple of years, the Army earlier this year reversed course and is trying to keep soldiers attached to the same unit for much of their careers.

Shay cautiously applauds the Army's recent shift, saying it goes in the right direction. But, Shay said he is not sure how far the service has moved or how permanent the changes will be.

"Changing the culture of a large institution is a very protracted process," Shay cautioned.

He said he sees some worrisome signs in the U.S. military in Iraq. The abuse of detainees at the Abu Ghraib prison, he said, directly resulted from the failure of leadership at the small unit level.

"What you need is a crusty, old sergeant who says at the right moment, 'We're soldiers; we don't do that [expletive],' " Shay said.

An Army report released in March found widespread problems with unit cohesion in Iraq. Its authors recommended that the Army do a better job of getting mental health resources to the troops.

Shay appears to disagree with that view.

"Honestly, I don't think the most important thing to do is to provide mental health professionals," he said. Rather, he returned to his three core issues: "The most important thing to do is to provide cohesion, leadership and training."

Overall, he said he is less worried about the mental health of regular, active-duty soldiers serving in Iraq than he is about the part-time troops in the National Guard and Reserves, and even more the thousands of private security personnel working on contract there.

Unlike service personnel, he noted, contractors have no formal network of support to help them when they return home, even if they are hired as bodyguards or placed in other combat-type roles.

"The amount of potential dynamite we are sowing in our own society by sending people into that situation, that way -- it just terrifies me," Shay said.

What do his friends and neighbors back in the liberal suburbs of Boston think of him helping the top brass make the Army more militarily effective?

"They know that I'm not just trying to turn people into more effective killers," Shay said. "The point of fighting in a just cause is to win, not to kill. The highest form of military skill is to win without killing."

What is more, he said, it also has to do with the morality of our own society. "If we're sending people to fight in our name," Shay said, "we damn well better be sure to win swiftly, and not kill any more of the enemy than is necessary."

"The point of fighting in a just cause is to win," says Jonathan Shay, just hired as an adviser to the Army's personnel chief. "The highest form of military skill is to win without killing."