How did two flawed human beings, assisted by two brilliant and determined diplomats, alter the course of history and set the world on a path to global interdependence and previously unimaginable economic growth? This is the question Margaret MacMillan sets out to answer in “Nixon & Mao: The Week That Changed the World” (Random House, 432 pages, $27.95) — her quirky, often wonderful account of President Nixon’s fateful 1976 trip to China.

As in her previous book on the Versailles peace conference, “Paris 1919,” Ms. MacMillan propels “Nixon & Mao” with her estimable gift for characterization. She has located the key to each of the personalities she writes about, and the reader will finish this volume with a well-rounded sense not only of the principals and their consorts Henry Kissinger and Chou En-lai, but also a large cast of supporting players. Ms. MacMillan strengthens her narrative with atmospheric details and telling anecdotes. One of the more memorable concerns a rude joke played on a young Barbara Walters by rival journalists in the traveling party: They dumped their dirty underwear on her bed, which was then delivered to her by a hotel attendant in a “very smelly package” while she was waiting in line to say goodbye to Chou.

Much of the material on Nixon is familiar — his shyness, social awkwardness, combat readiness, and, above all, his raging paranoia about his enemies. But Ms. MacMillan admires his genuine dedication to improving the condition of the world and his largely successful attempt to bring the world’s largest nation out of isolation, triangulate the Soviet menace, and substantially recalibrate international relations.

If Ms. MacMillan had done nothing more than rebut the notion that Nixon’s foreign policy was all Mr. Kissinger all the time, her book would have been an important contribution to historiography, not to mention bitter medicine for that legion of Nixon haters who simply cannot accept that he ever got anything right. Clearly, Mr. Nixon was the unmoved mover in the Chinese gambit, while Mr. Kissinger, deeply skeptical at first, developed the tactics to get the job done.

Ms. MacMillan’s portrait of Mao is deeply unsettling and calls to mind some of the archival material that has been leeching out in recent years on Josef Stalin. She has an entirely sound understanding of communism in theory and in practice, and there is never any hint of moral equivalence between that system and our own. Mao comes off a truly harrowing: When his mad economic schemes came to grief and the people were starving, he reportedly said, “Best halve the basic [food] ration, so if they’re hungry, they have to try harder.” When his doctor suggested that he refrain from sexual relations while being treated for venereal disease, his answer was “Not!” “If it’s not hurting me,” he said, “then it doesn’t matter.” As far as hygiene was concerned, Mao’s solution was more sex. “I wash myself in the bodies of my women.”

The torture and death of millions meant nothing to Mao, and he only called an end to the murderous Cultural Revolution when it became evident that Army training and readiness were suffering. And yet this wily peasant, who never grew accustomed to indoor plumbing, was a survivor whose instincts told him his 4,000-mile border with the Soviet Union was Problem No.1.

Fortunately, the increasingly reclusive Mao retained the services of Chou , the most enigmatic figure of Ms. MacMillan’s quartet. Charming, urbane, and well-educated, Chou was almost unaccountably a slave to Mao’s every whim. Ms. MacMillan locates his subservience in his unquestioning dedication to the revolution, his sense that he was a better adviser than leader, and his own nimble survival instincts in a bizarre and dangerous palace environment. Unhappily for Chou, nothing could save him when Mao forbade him a necessary cancer operation until it was too late. But by then, he had already wrapped up the Shanghai Communiqué, which set America and China on a path of further cooperation.

Despite all this rich material and the lively telling, there are a few startling lapses in this book that bear mentioning. In praising Nixon’s foreign policy credentials, Ms. MacMillan offers this: “Of all American presidents, with the exception, perhaps, of Bill Clinton, he [Nixon] had the best grounding in foreign relations.” With no supporting arguments and no elaboration, the reader’s mind fairly boggles — so much for Thomas Jefferson in France. Although she resists the temptation, embraced by so many these days, to predict the inevitability of Chinese predominance in coming years, she comes perilously close on a few occasions.

Still, this is a valuable book, not least because of the author’s keen sense of irony and the law of unintended consequences. One of the chief sticking points in the negotiations was the language relating to trade and other contacts. The Chinese were not much interested in increasing trade and tourism at the time, but Mr. Kissinger insisted, explaining that America needed such window-dressing words even though “we both know that basically they don’t mean anything.” As Ms. MacMillan rightly notes, the perspective 30 years later is that this was the most significant item in the agreement: Wal-Mart’s imports from China alone amount to $18 billion a year, 900,000 Americans travel to China annually, and a good many of China’s new leaders not only have degrees from American universities but a better understanding of the world beyond their shores.

Mr. Willcox is a writer and editor in Ridgefield, Connecticut.