Seventy miles south of the Arizona border, the isolated copper-mining town of Nacozari sits, like a lone Yucca tree, in the solitude of the sweltering Mexican desert. Last June, it was the scene of an act of harsh repression that will besmirch Mexico's image for years to come as Latin America's largest democracy.

Driven by despair over squalid driving conditions and low wages, some 3,500 workers at the mines owned by Mexicana di Cobre, a giant combine heavily financed by U.S. banks, walked off their jobs last Feb. 26. Two months later the Mexican government ordered soldiers and federal police to break the strike. The troops invaded the homes of 38 suspected strike leaders, who were bound, gagged, beaten and hauled into jail.

Production has since resumed, but the resentment smolders.

The ugly episode gained little notice in the press. We sent our roving reporter, Hal Bernton, to Nacozari for a first-hand look at the problems besieging Mexican mine workers. Many of them, he found, fear that their government has launched a strong-arm policy to brutally beat down a mounting labor unrest.

"We are living in a state of siege," one miner told Bernton. "We cannot talk because some of the federal police are disguised in miner's clothes. It's like Pinochet's Chile or Somoza's Nicaragua. There is no one we can turn to for support. We are betwen the sword and the wall."

After being released on bail, the seized strike leaders were banned from the mines. Many went into hiding to escape further police harassment; others returned to the barrio of cardboard shacks on the outskirts of town, where they live with their families.

"We want to be treated like human beings and not animals," said a 42-year-old strike participant who lives in a cardboard hovel with his wife and five children. "If the government continues to act as it has," he warned, "we shall all become communists."

Just behind his shack stood a chain of weathered wooden box cars that the miners have converted into primitive housing units. The decision to strike, said the miner, had been a difficult one. It was made after working three years, 12 hours a day, six days a week, to open what will soon become one of the largest copper mines in Latin America. Although a skilled construction worker, he was paid only $32 a week for his toil - less than the salary of a factory worker in Mexico City.

"What do I have to show for my efforts?" the leathery skinned miner asked. He pointed to the cramped two-room shack where his family of seven lives. There were three beds inside. On a kitchen shelf was the family's meager food supply - a few bags of beans, some noodles and a box of corn flakes. His children are forced to attend overcrowded schools, and the hospital lacks adequate medical facilities.

Single men who work for Mexicana di Cobre are crammed, 32 to a dormitory room, into hastily erected barracks that cling to the steep slopes of a valley. There are few trees on the barren site to provide shelter from the blazing desert sun. In winter, they complained, they lack blankets to keep them warm during the cold nights. They are transported to and from work in open trucks that allow little protection from the chilly winter rains.

Said one engineer who works with an American construction firm at the mine: "There is a high turnover rate which could be stopped if the company paid higher wages. We bring a man up from Mexico City, train him, and then he leaves because he doesn't have a good meal."

The Americans, company executives and a few privileged workers dwell in comparative comfort in company-built homes, which sit on a mountainside high above a poverty pockets of Nacozari.

That was the stage setting when the miners rebelled and appealed to the government mine workers' union for help. They then learned that the union had signed a secret contract with the company in December 1977. The union refused to divulge the terms of the contract.

A week after the work stoppage, a representative of the strikers met with Mexican President Lopez Portillo. He seemed sympathetic but never spoke out on the problem. Instead, the Portillo government bowed to pressures brought by Mexican industralist who own interests in the mine.

However, the military commander of the district, say our sources, refused an order to move in on the strikers. He was replaced by Gen. Alfonso Rangel, a hard-nosed officer known for his tough tactics against Mexican drug traffickers. On June 21, Rangel's assault force of soldiers broke into the homes of strike leaders.

"A soldier came into my home and put a rifle to my little girl's face and asked where I was," said one of the miners who was arrested. "When I arrived home, my girl was crying, yet she could not say why. Then the soldier came back and began to beat me in front of my children."

There is savage irony in the Portillo government's treatment of the strikers. In 1906, a group of outraged copper miners struck in the nearby town of Cannea. Repressive tactics by the Mexican government and U.S. mining interests helped to trigger a national revolution. The Portillo government boasts that its origins are rooted in that revolution. The government reverently proclaims that Cannea strikers as national martyrs.

The strike leaders at Nacozari fiercely believe that the Portillo regime is now betraying the revolution it so enthusiastically hails. As one miner grimly put it, pointing to the shanty town where he lives: "How is it possible that one of the richest mines in the world can have such a cordon of misery lying around its waist?"