Washington ? Two vastly different public officials — Robert McNamara and Sarah Palin — shared the spotlight this past week, triggering fresh thoughts about one of the classic dilemmas of governmental careers: When and how do you quit?

McNamara, the defense secretary for John Kennedy and Lyndon Johnson and later the president of the World Bank, died at 93, ushered out by lengthy obituaries recalling the controversies of the 1960s, including the furor over his unexplained “resignation” at the height of the Vietnam War.

A few days earlier, Palin, the 2008 Republican vice presidential candidate, had stunned her state of Alaska and the entire political world by announcing that she was leaving the governorship 18 months before the end of her term.

More opposite characters you could not imagine. McNamara, the owlish, frighteningly smart, statistically sophisticated, emotionally fragile executive, was the coldest fish in the exotic aquarium of the New Frontier.

Palin, the most colorful and charismatic figure to pop up in the GOP since Ronald Reagan, has displayed in equally vivid terms her brimming self-confidence and her striking ignorance of public policy.

Had they ever met, they would not have known what to make of each other. But they teach a couple important lessons about the way to handle exits from high office.

McNamara, the principal architect of the American buildup in Vietnam, was slow to recognize that escalation would not yield victory. But by 1967, his sixth year at the Pentagon, he had come to the conclusion that putting in more troops and expanding the air war against North Vietnam would not halt the Viet Cong. He wrote Johnson privately that it was time to negotiate with the enemy — and thereby end the growing domestic division over the war.

Johnson would not hear of it. He summoned McNamara to the White House and while the weary defense chief later commented that he was never sure whether he had resigned or been fired, he silently accepted Johnson’s decision to send him to the World Bank.

It was not until 1995, when he was again a private citizen, that McNamara published an apologetic memoir, revealing for the first time that he had harbored the gravest doubts about the war that took 58,000 American lives. The public reaction was harsh. Opponents of the war said that if McNamara had made the reason for his “resignation” public at the time, Johnson might have been forced to end the war — and thousands who died in the next seven years might have been saved.

McNamara said he had never seen himself in the role of a whistle-blower. As an appointee of the president, he said he owed Johnson his loyalty. The voters had chosen Johnson; his judgment deserved deference. It foreshadowed the similar decision by Secretary of State Colin Powell not to go public with his reservations about the Iraq War.

These are hard calls, and those of us on the outside, who can only imagine the pressures of public office, can show some sympathy for the people who have to wrestle with the conflict between their conscience and their sense of obligation to the administration in which they serve.

But resignation on a matter of principle is never a bad thing, and it can have salutary effects. This country would be better off if it happened more often.

By comparison, Palin’s decision to abandon her gubernatorial duties is much harder to understand or to justify. Whether this is a prelude to a presidential bid in 2012 or preparation for a high-paying media career or just a return to private life in Wasilla, the puzzlement and derision Palin has encountered are well-justified.

One of the traditional values for which conservatives are supposed to stand is: Finish what you started. Meet your responsibilities.

This looks like a wholly selfish decision on her part, with no large principles at stake.

McNamara stayed too long and left too quietly. Palin is bailing out on her people far too soon. Neither can serve as an example for those in government wrestling with the decision when to quit.