Juan Gonzalez, 12 and born a citizen of the United States, met Monday through a fence with his mother, Carmen Castillo, who was deported to Mexico in September after living in California illegally for 18 years.

In the heavily polluted New River near the California border, Mexican migrants plunge through filth, knowing the Border Patrol will not wade in to arrest them.

MEXICALI, Mexico (By James C. McKinley Jr.) March 23, 2005 - When United States Customs officials discovered the latest tunnel under the border here last month, they were stunned. With a cement floor and an intercom system, the passage ran nearly 200 yards from a house on one side of a rusty metal fence, under two streets and an apartment complex, to emerge in an unassuming tract home in California.

Though more elaborate, the tunnel is not unlike the 13 others found during the 1990's, built by drug cartels. But everything in the world after Sept. 11, 2001, has taken on a different hue. Today such tunnels are where the failures of drug policy, border control and immigration reform meet ever pressing issues of national security. American officials fear the tunnels could be used just as easily to smuggle terrorists and explosives as cocaine or illegal immigrants.

That confluence of worries forms the backdrop for a meeting on Wednesday in Texas between President Bush, President Vicente Fox of Mexico and Prime Minister Paul Martin of Canada. But where issues converge, the interests of the United States and its neighbors may not.

For Mr. Bush and Congress, security tops the agenda. For Mexico, it is a freer flow of migrant workers. For Canada, it is the imperative of foreign and domestic policies that increasingly diverge from Washington's conservative consensus.

Senior Bush administration officials said Tuesday that the leaders were not expected to announce any concrete agreements after the one-day meeting at Baylor University in Waco. Instead, the men will announce a new framework and timetable for resolving a host of sticky trade and security issues, among them letting more workers cross borders legally for jobs and improving cooperation against terrorists.

A year ago, Mr. Bush proposed greatly expanding a guest-worker program for Mexican laborers. But to Mr. Fox's great dismay, the idea has faded. In the last two weeks, American diplomats have made it clear that Congress is unlikely to act unless Mexico does more to tighten up the border and reduce the rampant crime on its side.

"What Mexico needs to understand is that migration is viewed largely as a security issue in the United States, and they appear to think that that is not as important as we do," said Senator John Cornyn, a Texas Republican who has taken a lead in the debate on a guest-worker program.

Perhaps nowhere is the inexorable nature of the northward migration of Mexicans - and the vulnerability of the United States to infiltration, whether by migrants or by terrorists - more apparent than in Mexicali and in its sister city, Calexico, Calif.

Investigators say they doubt that the builders of the elaborate tunnel here would have spent an estimated $1 million just to smuggle migrant workers. It is more likely, they said, that the tunnel was built to smuggle lucrative drugs like cocaine and heroin, but another line of investigation is that its builders might have intended to sell passage to terrorists.

"You have this gaping hole that could be used for anything," said Michael Unzueta, the acting special agent in charge of the San Diego office of Immigration and Customs Enforcement.

Meanwhile, the alarms have been sounding in Washington about the dangers post-9/11 of a porous, 2,000-mile-long border. James Loy, the deputy secretary for Homeland Security, said last month that intelligence reports showed that terrorists from Al Qaeda were likely to try to enter the country from Mexico, across whose border at least 300,000 people flow every year virtually untraced and with impunity.

Porter J. Goss, the director of central intelligence, told the Senate Armed Services Committee last week that the United States was vulnerable to terrorists infiltrating through its backyard.

But for President Fox, a tighter border that keeps Mexicans from desperately needed jobs is not necessarily in his interest. Last week, in a sharp divergence from Washington, he publicly decried a measure passed in the House of Representatives that would mandate completion of a long-stalled security wall between Tijuana and San Diego.

Far from being completed, he said, the wall should be knocked down. "No country that is proud of itself should construct walls," he said.

He and other proponents of immigration reform and the guest-worker program argue that tighter controls will do more harm than good, by forcing more migrants to take illegal routes, and thus making it easier for terrorists to cross illegally as well.

But opponents, like Congressman Tom Tancredo, a conservative Colorado Republican, say a guest-worker program makes no sense unless there is already a tight border.

In increasingly strident language, Mr. Fox has painted the anti-immigrant movement as the work of "minority, xenophobic, discriminatory groups" who do not recognize the contributions of the three million Mexicans who work illegally in the United States. The House measure passed last week would make it harder for immigrants to get a driver's license.

More alarming, a group of self-proclaimed patriots, calling themselves the Minuteman Project, are planning to set up a vigilante watch along the Arizona border in April to report illegal aliens.

Here in Mexicali, the tunnels are just one route across. Every morning Border Patrol agents find five to eight new holes sawed in the rusty metal fence that divides the city from the United States. Albert Garcia, a Border Protection welder, goes dutifully along the barrier, putting back together the heavy square tubes with a spray of sparks and fire.

At night, migrants strip to their underclothes and slip into the fetid water of New River, a polluted waterway that smells of feces, chemicals and all other manner of putridity.

They float by silently in clusters amid odd patches of white foam caused by detergents, while Border Patrol agents watch from the shore, waiting to see where they will try to get out and run. "They know we won't go into that water after them," said one agent, who did not give his name. "It's not worth the risk."

Others vault the wall, falling to ground on the other side then sprinting to a waiting car, or cross the frigid desert highlands or the sweltering Sonoran desert.

About 80 migrants die on the American side each year from exposure just in this area, customs officials said. Last year, the Border Patrol picked up 1.1 million people trying to cross illegally; estimates of those who get through range from 300,000 to 400,000, officials said.

For some Mexicans, the border here seems a cultural affront. On Monday evening Carmen Castillo, a 47-year-old former nursing aide who along with her husband was deported last September after living illegally for 18 years in California, came to the Mexicali wall to visit her five children and a grandchild she had never met.

She talked to her children, all of whom were born in the United States and have citizenship, playing with the new baby through the rusty bars dividing the countries.

"It's like visiting in prison," a daughter, Carmen Nero, said as she held her infant son up to the bars. "It's heartbreaking. It's sad that there's a fence when we know we are all supposed to be together."