自 1960 年以來，美國便貿貿然增兵越南，卻毫不了解越南曾是法國殖民地，以及當地反抗帝國主義的民心。直到 1975 年 4 月 30 日福特總統（Gerald R. Ford）任內美軍完全撤出越南為止，美軍共 58,159 人死亡、2,000 人失蹤、303,635 人受傷。1993 年，McNamara 曾在接受訪問時公開承認了越戰的錯誤：「甘迺迪和詹森政府依照我國的原則和傳統行事，但是我們錯了，大錯特錯。」（"We of the Kennedy and Johnson administrations acted according to what we thought were the principles and traditions of our country. But we were wrong…"）
1995 年，McNamara 出版回憶錄 In Retrospect: The Tragedy and Lessons of Vietnam《回顧：越南的悲劇與教訓》。這部回憶錄毀譽參半，有人認為 McNamara 試圖以一種道歉的手段將越戰的責任往別人身上推，藉以粉飾其戰爭發動者的真面目。
McNamara 在他的回憶錄中揭櫫的越戰徒勞無功的十一個原因：
（"Eleven lessons from the Vietnam War"）
- We misjudged then — and we have since — the geopolitical intentions of our adversaries … and we exaggerated the dangers to the United States of their actions.
我們誤判了對方的地緣政治意圖，並向自己誇大了對方行動的危險性。 - We viewed the people and leaders of South Vietnam in terms of our own experience … We totally misjudged the political forces within the country.
我們以美國的經驗衡量南越的人民和領導，誤判越南境內的政治力量。 - We underestimated the power of nationalism to motivate a people to fight and die for their beliefs and values.
我們低估了對方民族主義的力量。 - Our judgments 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.
我們對越南歷史文化政治及其領導人深刻無知。 - We failed then — and have since — to recognize the limitations of modern, high-technology military equipment, forces, and doctrine.
未能認清現代化高科技軍備、軍隊、和軍紀的限制。 - 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.
對方的文化與我們截然不同，我們未能將戰術運用於贏得對方民心。 - 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 military involvement … before we initiated the action.
未能在開戰前邀集美國國會和人民針對此一大規模的戰事進行全面坦白的正反辯論。 - After the action got under way, and unanticipated events forced us off our planned course … we did not fully explain what was happening, and why we were doing what we did.
開戰後，始料未及的事發生時，並未充分向國人解釋。 - We did not recognize that neither our people nor our leaders are omniscient. Our judgement 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 image or as we choose.
未能認清我們並非無所不能的國家。我們判斷何以為別人國家的最佳利益應在國際論壇上公開討論。上帝沒賦予我們權利照我們的想法和選擇去塑造每個國家。 - We did not hold to the principle that U.S. military action … should be carried out only in conjunction with multinational forces supported fully (and not merely cosmetically) by the international community.
我們未能把握這樣的原則：只有國際社會完全（不僅是表面）的支持下，連同多國部隊，我們方可進行這樣的軍事行動。 - 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 … At times, we may have to live with an imperfect, untidy world.
我們未能認清，國際事務，正如日常生活，有許多問題不是立即可解決的...有時，我們就是得生活在一個不完美而雜亂的世界。
