A week ago Sunday, a weary but relieved U.S. Ambassador Arthur Hummel Jr. shook hands with Chinese officials across a green, felt-topped negotiating table and set Sino-American relations back on course after months of strain.

Chinese and American diplomats had been struggling over endless cigarettes and cups of tea since October, trying to solve what seemed unsolvable -- Washington's military commitment to Taiwan versus Peking's irredentist claim to the capitalist island.

But, when Hummel left the negotiating session, he had an agreement that drew China and the United States back from the edge of diplomatic breakdown, keeping open the way for a strategic relationship between the world's most powerful and most populous nations.

For Hummel, 62, an angular, plain-talking man who moved from an adventurous youth into the measured world of diplomacy, it was just the latest skirmish in an internal Chinese "civil war" that he has been waging on and off for most of his life.

Appointed ambassador last August, Hummel brings to his post a personal background that is unique in the U.S. Foreign Service. Beginning as a self-described "missionary brat" born in China, he went on to fight with Nationalist guerrillas during World War II, and, as a U.N. relief official, gained firsthand knowledge of the conditions that led to the Communist victory in 1949 and the flight of the Nationalists to Taiwan.

From his early days, he fashioned himself as a kind of renaissance man, hitchhiking across America's Midwest, taking odd jobs as a private detective and factory worker before entering government service. Even as a diplomat, he has had a colorful background, having launched the U.S. war against Burmese opium, saved his staff from a burning U.S. Embassy in Pakistan and faced down an intimidating Henry A. Kissinger at the State Department.

Yet, Hummel's flamboyant years outside China seem like an accidental prelude to his current role, for, as a fellow ambassador to Peking says, "China is the center of his life."

The diplomatic colleague, who has known Hummel for 30 years, said the U.S. ambassador "has an understanding of its history, the Chinese way of doing things. This gives him a special perspective."

Hummel brought the perspective with him to the negotiating table in recent months, apparently using it to his advantage. His comprehension of Chinese gave him extra time to formulate his responses while the English translator worked. His familiarity with Chinese officials made it easy to slip away from the intense sessions for an informal chat over lunch, when, he said, "We made progress and gave each other clues."

"The personal dynamic helped in covincing the Chinese of the American reasons" for selling Taiwan weapons, "but how much that affected their decision-making, I don't know," he said in a recent interview. "Personal acceptability is one thing, and foreign policy is another. I can't claim that this rubs off in government decisions to please an old American friend."

Underscoring the seriousness of the Chinese threat to downgrade relations, Hummel said he felt the "cold breath on my neck." At the same time, Hummel's arguments about the demands of American policy and politics in relation to Taiwan were driven home by visiting top-level U.S. officials, who made it clear to the Chinese that no accord could be reached without congressional agreement that the demands of the Taiwan Relations Act had been met.

In the American view, approved at the highest levels of government, this required that a U.S. commitment to reduce sale of arms to Taiwan be matched by a long-term Chinese commitment to a peaceful resolution of the reunification issue. This was met when Peking came up with the formulation that peaceful settlement of differences with the island was China's "fundamental" policy.

Hummel was born in Shanxi Prvince in 1920 and spent his first eight years in Peking, learning fluent Chinese. His father, Arthur Hummel Sr., was a Congregationalist minister and noted Sinologist whose history of the Qing Dynasty still is considered a standard in the field.

After his father moved to Washington, where he headed the Orientalia Division of the Library of Congress, Hummel became a rebellious youth, twice thrown out of prep school. He dropped out of Antioch College and, by his own description, lived like a hippie before it was fashionable.

Drifting from job to job, he decided to go back to China in 1940 and was teaching English in Peking when the Japanese bombed Pearl Harbor in December 1941. Japanese troops then occupying Peking rounded up Hummel with other enemy aliens and put him in an internment camp for the next two years.

By 1944, Hummel had managed to escape the prison camp in Shandong Province with the help of a Nationalist Chinese guerrilla band then fighting a three-cornered war against the Japanese, their Chinese puppet troops and Chinese Communists, who were gaining ground in rural areas.

Hummel joined the guerrillas, and for the next 15 months he fought Japanese and puppet Chinese troops along the flat, dusty Shandong plain. By day, his rag-tag outfit hid from brutal Japanese mop-up operations, resurfacing at night for sudden ambushes.

"It was exciting," he said of the experience that would reshape his life. "I enjoyed it immensely." It was during his days as a guerrilla fighting under the nom de guerre Hong Anshi when Hummel learned the power of Chinese nationalism, he recalls today. He carried the lesson with him to the negotiating sessions in Peking 40 years later.

"Nationalism is a very powerful factor," he said. It was the [Chinese] insistence on sovereignty that was the problem of Taiwan, and that's basically a nationalistic element."

Newly motivated after his wartime experience, Hummel worked for a year after the war as a U.N. relief officer surveying Communist-controlled areas of China's northeast and then returned to the United States to take a graduate degree in Chinese from the University of Chicago.

He joined the State Department in 1950 and put his China background to work right away. High-ranking U.S. diplomats were embroiled in McCarthyite charges of selling out China to the Communists, and the new staffer helped prepare their defense.

Climbing the bureaucratic ladder, he became the number two man at the U.S. Embassy in Taiwan from 1965 to 1968, then for the next three years, ambassador to Rangoon, where he determined that large quantities of opium were being smuggled out of Burma.

Back in Washington, Hummel was acting assistant secretary of state in 1973 when he helped Chinese officials set up a liaison office, the first step in the lengthy normalization process. The Chinese delegation was headed by Han Xu, a suave, English-speaking diplomat who became the chief negotiator for Peking in the just-concluded talks.

During 1973, Hummel emerged as adviser on China to then-secretary of state Kissinger. Washington was sorting out the China question at the time, and the two men did not always agree.

"To this day, Henry likes to tell people that I'm mean to him," Hummel said with amusement. "I'm proud of the fact that I'm one of the few Kissinger subordinates who, after one of his statements, looked him in the eye and said, 'bullshit,'" he recalled.

"Four or five times he directed me to do things I just thought were wrong. In shouting matches, I insisted that at least he listen to me.

"Sometimes, Kissinger would listen, but I ended up doing what he wanted anyway. Frankly, I believe that's the only way to deal with one of these mad geniuses, which is what he is. There was a daily sense of tension whether you were going to have your job the next day."

Kissinger never fired Hummel, but he dispatched him to Ethiopia, where he served as ambassador while the ruling leftist government gradually swung toward the Soviet Union.

Hummel's star began to rise again after president Carter sent him to Pakistan, where he was head of mission when Islamic radicals set fire to the U.S. Embassy in 1979, trapping several dozen American staffers and visitors inside a protected vault.

Hummel, who had been at home during the siege, called Pakistani President Mohammed Zia ul-Haq and demanded protection for his staffers.Only minutes before it would have been too late, an Army helicopter arrived and plucked the gagging Americans from the roof of the embassy.

Hummel believes it was his success in bringing Pakistan more firmly into the American orbit by putting together a $3 billion aid package in 1981 that convinced then-secretary of State Alexander M. Haig Jr. to give him the coveted China posting.

To his admirers in Peking, it seemed like the natural assignment.

"He's absolutely unflappable," said a U.S. Embassy official who has watched Hummel operate. "He understands the Chinese negotiating techniques and the meaning behind the barrage of words."

But Hummel, who grew up with modern China, believes too much can be made of expertise in untangling the China puzzle.

"These people are not very different from anyone else," he said. "The rules of the game are a little different, but once you understand the social milieu, there's nothing very esoteric about dealing with Chinese."