How tough a border?

Despite the emphasis on strong enforcement of the US-Mexican border, there is relatively little harshness in the relationship between US agents and millions of illegal aliens caught each year ON a public square in Nuevo Laredo, Mexico, a statue of a Mexican war hero stands above a plaque reading, ``Mi patria es primero'' -- My country is first.

The irony is that just a short walk north flows the Rio Grande, the river that hundreds of thousands of Mexicans cross every year in hopes of finding employment in the United States. Whether they cross the Rio Grande to Texas, or the deserts of California, Arizona, or New Mexico, they are saying with their feet: My family's needs, my own desire for economic stability, are first.

It's a northward march that has gone on for more than a century. But the number of marchers has multiplied as galloping population growth and the abrupt change in Mexico's economic structure -- from an agrarian to an urban industrial society -- have increased the attraction of ``El Norte.''

The number of apprehensions of illegal aliens has grown every year since 1960. For the past three years, the numbers have topped 1 million, about 95 percent of whom are Mexican. It's a situation that has garnered increasing and ever-more-emotional attention as everyone from local school boards to Congress debates the effect of illegal immigration on jobs, services, the economy, and ultimately the American way of life.

The US Border Patrol has been beefed up as one means of stemming the flow. A 40 percent increase in the patrol's budget last year exemplifies the importance the Reagan administration places on regaining control of the nation's borders.

That increase, from $116 million to $156 million, put about 850 new agents on the southern border. Of the patrol's 3,425 personnel, 3,150 are assigned to the Mexican border.

Just how much plugging-up these agents can do in what has come to be considered a porous barrier is open to question.

``They can slow it down, but you have to look at the qualities of the people coming here,'' says Rodolfo de la Garza, an authority on Mexican-American affairs at the University of Texas at Austin. ``They are energetic and they are determined.'' He adds that since the risk of crossing and getting caught is no greater than ``staying there with nothing,'' any plausible increase in border patrols is unlikely to reduce the flow significantly.

Yet if nothing else, the patrol and its increasingly sophisticated equipment are making illegal border crossings a riskier business.

Units of the patrol in Arizona are using infrared detectors that can sense the warmth of a human body more than two miles away. In McAllen, Texas, where 18-wheel trucking rigs are the latest means of transporting large numbers of aliens, the patrol has tested a device that detects the slight motion of breathing.

``We've found up to 80 aliens in a sealed truck,'' says Silvestre Reyes, chief patrol agent for the McAllen sector. ``They pile produce or other materials around them, then set out on their journey north.''

Border agents estimate that with 2,000 miles of border to watch and the growing volume of legal crossings -- more than 1 million a day -- handled at the border's 24 checkpoints, perhaps two aliens slip through for every one stopped. Not everyone accepts that estimate, however. Many border experts believe that, as the border patrol grows, apprehensions include an increasing number of repeat offenders and therefore do not accurately represent the numbers of people trying to enter the US.

Yet border agents insist that while there are repeat offenders, sometimes being nabbed four or five times in one day, there is no doubting the increase in aliens attempting to cross.

``Back when I joined the patrol in 1960, if you caught one [alien] in a two-week period, it was a lot,'' says Larry Teverbaugh, chief agent of the Laredo sector. In October this year, his 200 agents apprehended 10,283 undocu-mented aliens (a 50 percent increase over October 1984). They also arrested 97 people for smuggling aliens.

With apprehended Mexicans routinely escorted to the other side of the border and released, often to turn around and try again, the process can begin to resemble a game of cat and mouse.

But crossing can be a dangerous enterprise for aliens who run into ``border bandits.'' At the least, they risk losing the life's savings they have tucked away in a shoe, but many are maimed or killed. San Diego estimates it spends $200,000 a year protecting illegal crossers.

``There is a lot of violence, much of it never detected,'' says Mr. Teverbaugh. He adds that drowning, exposure, and simplistic assumptions carried along from home are among the other dangers the undocumented aliens face.

In July alone, the Laredo sector counted 11 alien deaths, nine involving trains. ``They sleep on the tracks where they feel safe from the snakes,'' says Teverbaugh, and they assume that, as in the rural areas most of them call home, there will be no trains before dawn. ``They're so tired from their journey,'' he adds, ``that when the night trains do come, they never even wake up.''

But still they come, and still the debate continues in the US on the issue of illegal immigration.

Omer Sewell, district director for the Immigration and Naturalization Service (INS) in Harlingen, Texas, says that as a young border-patrol agent in the early '60s he had a simple solution to the problem: ``Arrest all violators and throw them out of the country.''

But he says experience has taught him that the issue is more complex, involving economic factors that defy quick responses. Like most INS officials, he favors sanctions against employers who take on undocumented aliens. ``It doesn't make sense that it's illegal for someone to come here to work, but OK for an employer to hire as many of those illegals as he wants,'' says Mr. Sewell.

Yet he recognizes ``a definite need for some of these workers,'' and the major role generations of alien laborers have played in building the southwestern United States.

Many researchers believe an aging US population and an expanding service sector with growing numbers of low-paying jobs mean that within a decade the country could face labor shortages that would shift the focus on alien workers. ``If unemployment hits 4 or 5 percent, you'll see illegal immigration disappear from the national agenda in a snap,'' says Gilbert Cardenas, an economist with Pan American University in Edinburg, Texas.

In the meantime, conditions in Mexico will continue to push people north.

``Like the salmon, we're born with the river in us,'' said one Mexican-born American in San Diego.

``Some of us will die, but we'll make the swim. In the hierarchy of needs, food, water, and shelter are on top. That's what moves us.''