“War is so complex it’s beyond the ability of the human mind to comprehend,” wrote former defense secretary Robert S. McNamara. “Our judgment, our understanding, are not adequate. And we kill people unnecessarily.”

That realization came late in life for Mr. McNamara, who died Monday at the age of 93. As defense secretary during the Kennedy and Johnson administrations, he was the architect of America's tragic misadventure in Vietnam, a decade-long conflict that even he was eventually forced to conclude had been a terrible mistake. But by then the war already had claimed  the lives of 16,000 Americans, and 42,000 more would die before it finally ended seven years later.

As the antiwar movement grew during the 1960s, McNamara became more and more personally identified with the war's costs in blood and treasure. But throughout that period he insisted publically that victory was at hand, even though privately he had begun to doubt both the military and moral justification of the war. It was not until 1995 that he finally admitted the war had been futile from the start and that America's involvement had been "wrong, terribly wrong." His critics excoriated him for his belated confession, and for the rest of he life he was haunted by the consequences of his failure as one of the country's "best and brightest."

Kennedy called McNamara the smartest man he had ever met. So how could such a brilliant leader and administrator -- before becoming defense secretary, McNamara had risen to president of  Ford Motor Co. and dramatically turned its fortunes around -- have erred so grievously in his prosecution of what turned out to be an impossible war?

The answer may lie in the classic formulations of Sun Tzu, the ancient Chinese military theorist, and Thucydides, the Greek historian who chronicled the decades-long war between Athens and Sparta. Both cautioned against embarking on wars lightly, since the outcome is always unpredictable; they also warned that knowing one's enemy is crucial to success. America ignored both those lessons in Vietnam, a conflict it entered on the basis of flawed intelligence and against an enemy whose motives it never clearly recognized or understood. One can only hope the country won't repeat those mistakes in Iraq and Afghanistan, where out enemies are equally determined to waste the blood and treasure of an entirely new generation of Americans.