He is short and fat, from a poor peasant family. For a time he earned a meager living as a temple monk and until joining the Communist Party in his early 20s he probably could neither read nor write.

But Hsu Shih-yu, reportedly commanding China's invasion of its former socialist ally Vietnam, has in 50 years of soldiering developed a reputation as a superb organizer and deft strategist, with scores of Chinese civil war victories under his belt.

Hsu, 72, makes a show of gruffness and plain talk. But he has become a canny party politician who has survived during periods when his impatience with the dogma of Chairman Mao Tse-tung might otherwise have spelled political doom.

"If there is a military tactician in China today who can handle the very best that Vietnam has, it is Hsu Shih-yu," said former U.S. Army Col. William W. Whitson, perhaps the leading American expert on China's People's Liberation Army.

Hsu's appointment as united front commander for the invasion, however, may have as much to do with his experience as military commander for the southeast region bordering Vietnam and with his long, close ties to China's number one strategist, Vice Party Chairman and Army Chief of Staff Teng Hsiao-ping.

The careers of Hsu, the stolid commander, and Teng, the brilliant political officer, crossed often in the 1930s and 1940s as the Communists gradually outfought the Nationalist Chinese led by Chiang Kai-shek. In the late 1960s, Hsu as commander of the Nanking region firmly resisted Red Guard attacks on established authority that had swept his friend Teng out of power. In 1976, when Teng suffered another brief purge, Hsu reportedly brought him to Canton, where Hsu had been transferred, to protect him from any physical harm by his political enemies in Peking.

That close personal relationship may be the key to a situation in which Teng wants to fight a finely tuned war that will punish the Vietnamese without forcing the Soviets to intervene. "Hsu will go as far as he is supposed to go, and when Teng says stop, he'll stop," one analyst said.

Hsu was born in Hupei. He joined the Communist Party with other peasant youth of that area who made up the early guerrilla bands set up by China's present defense minister, Hsu Hsiang-chien, who is no relation. By his late 20s he was commander of a small Communist division and joined in the Long March that took Mao's forces to northern China.

There, at the Communist base camp in Yenan, began the formal education in military tactics that Hsu now must employ against the Vietnamese generals who beat the French and the Americans.

Much of the credit for the tactical skills of Hsu and other Chinese generals goes to the Soviet Union, now firmly on the side of China's Vietnamese adversaries. The leading Chnese disciple of Soviet military strategy, still holding a seat in China's Politburo, is 87-year-old Liu Pucheng. Liu spent 2 1/2 years in the Soviet Union in the late 1920s and later set up officer schools at Yenan and, after the 1949 Communist victory, in Peking.

It was a heavily professional curriculum, influenced by Russian and German emphasis on strong general staffs and logistics. Its graduates eventually came into conflict with Mao's preference for guerrilla tactics, or "people's war."

Hsu fought the remainder of the civil war mostly in the Shangtung area, advancing south to mop up Nationalist resistance in coastal provinces toward the end of the conflict. About 1955 he became commander of the Nanking military region and remained there for two decades. He became so well connected and familiar with the command that ultraleftists in the Peking leadership failed in attempts to remove him during the Cultural Revolution of the late 1960s.

In January 1967, workers' organizations opposed to Mao's massive purge of the party demonstrated in Nanking. Despite screaming from Peking leftist, Hsu did little to stop them.

He resisted repeated calls to come to Peking for consultation, and perhaps removal from office, until late in 1967, when revolutionary ardor had cooled and his position was safer.

As the Cultural Revolution ended, he chided a group of former Red Guards being sent off to work in the countryside. They looked weak and pale, he said. The peasant's life would do them good. "Look at me," he said. "I'm over 60 but I don't need glasses like many of you. I can ride a wild horse and shoot a moving target target 100 yards away."

Hsu was elected to the ruling Poliburo and was widely expected to become defense minister some day, although the post is now held by his early mentor from Hupei. He is now commander of the Canton military region, which includes the Kwangsi Chuang Autonomous Region on the border with Vietnam.

Although Hsu has never been officially credited with service during the Korean war, there are great gaps in his record in the early 1950s that suggests he may have spent time there. If so, it was probably in a position junior to the man who now serves as deputy commander for the Vietnam operation, Yang Teh-chih.

Yang is commander of the military region including Yunnan, China's other province bordering Vietnam. Yang shares Hsu's humble beginnings as an illiterate peasant blossoming under the party's wing. Yang was deputy commander of the Chinese volunteers' in Korea, China's last military engagement of the scope of the Vietnam operation.

Yang, unlike Hsu, is not a Politburo member. But the invasion chief of staff, Chang Ting-fa, also the air force commander, is a member of the party's ruling body.

Peking has called the invasion a "counterattack" to pay back the Vietnamese for their attacks on the Chinese border. But some foreign onlookers wonder if Hsu and his soldiers are also being given a chance to test an army that has begun a crash improvement program after a decade of political turmoil and years without real military action.

"I can see Teng saying to those commanders, said one foreign analyst. "I know you want to modernize the military, but let's see first how good you are now."