"Sideshow," a recent book on Cambodia by the accomplished British journalist William Shawcross, provided Vietnam War critics with occasion for a new round of attacks on Henry Kissinger. Now Kissinger has struck back in a letter on the Shawcross book to The Economist.

As counterattack, the Kissinger letter is strong stuff. But from between the lines there emerges a line of questioning grounded not in the quaint conceits of liberal intellectuals about Southeast Asia, but in Kissinger's own concepts of legitimacy and world order.

The anti-Kissinger argument in the Shawcross book goes as follows: Kissinger was a major architect of the secret bombing of Cambodia, which the Nixon administration instituted in March 1969. That bombing drew North Vietnamese forces into the area.

Their entry into Cambodia set up the coup against the Cambodian ruler, Prince Sihanouk. Kissinger and Nixon used the ouster of Sihanouk as a pretext for the American invasion of Cambodia in April 1970. As a result, all-out war came to that peaceful country. There followed double devastation -- first at the hands of a Communist regime under Pol Pot and most recently through a Vietnamese invasion.

In his letter, Kissinger pulls apart that supposedly inexorable chain of events. He cites Sihanouk, whom he saw in Peking in April, as witness that the bombing did not draw the North Vietnamese any deeper into Cambodia.

He shows that the fall of Sihanouk surprised and dismayed Washington, and came about largely because of local circumstances. He demonstrates that the invasion of 1970 was at least in part undertaken because the North Vietnamese had been using Cambodia as a base for attacks on Americans in South Vietnam. He persuades me that even after the invasion there were plenty of chances for events to turn out differently.

Along the way, Kissinger identifies the motive that caused so many of the war critics to use the Shawcross book for revived attacks at this late date. He claims -- rightly, I think -- that the critics have felt obliged to shift the base of the argument to Cambodia because they have been embarrassed by the recent behavior of the Vietnamese Communists.

But a good offense is not necessarily the best defense. The secret bombing of Cambodia can be fitted into a much different reconstruction of events.

The former secretary of state acknowledges that the bombing of Cambodia was instituted in secrecy with a special system of false reporting. He thus implicity admits an element of duplicity.

The purpose of the secrecy, Kissinger asserts, was to spare Sihanouk the humililation of having to protest the bombing. When the Cambodian leader did not protest, and when the key senators raised no objections, the secret bombing was kept up.

To me, that practice suggests the belief that an element of cheating was okay as long as the traffic would bear it. It implies that the president, far from having an overwhelming responsibility for straightforward dealing, was entitled to do what he could get away with.

But where did that approach lead? Why, after all, did Nixon become involved in the "third-rate burglary" and cover-up that followed the Watergate break-in? Wasn't it really because he assumed he could get away with it?

More to the point with Kissinger is another line of questioning. What happened when the Watergate inquiry disclosed the cheating in Cambodia and in other matters? What did that do to the reputation of the president and his senior advisers in foreign policy? Didn't it weaken their credibility in the eyes of Congress and the nation? Didn't it erode their legitimacy?

Didn't the Congress, in consequence, institute a series of special measures that effectively tied the hands of the Nixon administration? Didn't the country, as a whole, come to distrust any action taken in the name of national security? And by any institution charged with special responsibility for national security?

Wasn't one result the feeble American response when South Vietnam and Cambodia started to fall? Wasn't another result the inability of American leaders to respond effectively to the far more serious challenges that have developed in the Middle East and the Persian Gulf?

Isn't the lesson of all this that very high stakes are required to justify even a little duplicity in high office? Don't the stakes have to be far higher than the marginal advantage of bombing a marginal part of a marginal country in a marginal war? And shouldn't that point be felt with particular force by persons who set store by legitimacy as a binding feature of world order?

I leave the answers to the reader. For my own part, I am persuaded that, at worst, the questions involve matters of judgment, not, as some aver, of crime.