IT is sometimes said that the post-Cold War world will be so different from the world of the past that the lessons of Vietnam will be inapplicable or of no relevance to the 21st century. I disagree. That said, if we are to learn from our experience in Vietnam, we must first pinpoint our failures. There were 11 major causes for our disaster in Vietnam:

1. We misjudged then -- as we have since -- the geopolitical intentions of our adversaries (in this case, North Vietnam and the Vietcong, supported by China and the Soviet Union), and we exaggerated the dangers to the United States of their actions.

2. We viewed the people and leaders of South Vietnam in terms of our own experience. We saw in them a thirst for -- and a determination to fight for -- freedom and democracy. We totally misjudged the political forces within the country.

3. We underestimated the power of nationalism to motivate a people (in this case, the North Vietnamese and Vietcong) to fight and die for their beliefs and values -- and we continue to do so today in many parts of the world.

4. Our misjudgments of friend and foe alike reflected our profound ignorance of the history, culture, and politics of the people in the area, and the personalities and habits of their leaders. We might have made similar misjudgments regarding the Soviets during our frequent confrontations -- over Berlin, Cuba, the Middle East, for example -- had we not had the advice of Tommy Thompson, Chip Bohlen, and George Kennan. These senior diplomats had spent decades studying the Soviet Union, its people and its leaders, why they behaved as they did, and how they would react to our actions. Their advice proved invaluable in shaping our judgments and decision. No Southeast Asian counterparts existed for senior officials to consult when making decisions on Vietnam.

5. We failed then -- as we have since -- to recognize the limitations of modern, high-technology military equipment, forces, and doctrine in confronting unconventional, highly motivated people's movements. We failed as well to adapt our military tactics to the task of winning the hearts and minds of people from a totally different culture.

6. We failed to draw Congress and the American people into a full and frank discussion and debate of the pros and cons of a large-scale US military involvement in Southeast Asia before we initiated the action.

7. After the action got under way and unanticipated events forced us off our planned course, we failed to retain popular support in part because we did not explain fully what was happening and why we were doing what we did. We had not prepared the public to understand the complex events we faced and how to react constructively to the need for changes in course as the nation confronted uncharted seas and an alien environment. A nation's deepest strength lies not in its military prowess but, rather, in the unity of its people. We failed to maintain it.

8. We did not recognize that neither our people nor our leaders are omniscient. Where our own security is not directly at stake, our judgment of what is in another people's or country's best interest should be put to the test of open discussion in international forums. We do not have the God-given right to shape every nation in our own image or as we choose.

9. We did not hold to the principle that US military action -- other than in response to direct threats to our own security -- should be carried out only in conjunction with multinational forces supported fully (and not merely cosmestically) by the international community.

10. We failed to recognize that in international affairs, as in other aspects of life, there may be problems for which there are no immediate solutions. For one whose life has been dedicated to the belief and practice of problem solving, this is particularly hard to admit. But, at times, we may have to live with an imperfect, untidy world.

11. Underlying many of these errors lay our failure to organize the top echelons of the executive branch to deal effectively with the extraordinarily complex range of political and military issues, involving the great risks and costs -- including, above all else, loss of life -- associated with the application of military force under substantial constraints over a long period of time. Such organizational weakness would have been costly had this been the only task confronting the president and his advisers. It, of course, was not. It coexisted with the wide array of other domestic and international problems confronting us. We thus failed to analyze and debate our actions in Southeast Asia -- our objectives, the risks and costs of alternative ways of dealing with them, and the necessity of changing course when failure was clear -- with the intensity and thoroughness that characterized the debates of the Executive Committee during the Cuban Missile Crisis.

* From ''In Retrospect: The Tragedy and Lessons of Vietnam'' by Robert S. McNamara. Reprinted with permission of Times Books, a division of Random House Inc.; copyright 1995.