Sgt. Brian Johnston was the duty officer at 6:15 a.m. when a call came from the Navy Shore Patrol: Two drunk Marines from his battalion had been picked up in Tijuana and needed a ride back from the Mexican border to Camp Pendelton in Southern California.

When Johnston arrived at the border about an hour later, he recalled, he became confused and ended up in a traffic lane that funneled him into Mexico, where he got out and asked customs police for directions to the Shore Patrol station. Johnston, 23, said he was unarmed but wearing a U.S. Marine camouflage uniform. He showed the Mexican customs officers his military ID, he said, and explained why he was there.

"Carrying any weapons?" one asked. Johnston unlocked the tool chest in the bed of his pick-up and showed them a rifle and a pistol: his own, nonmilitary weapons. "You are in big trouble," another said.

"They pointed to a building on the other side of the border and said, 'That's where you need to go,' " Johnston said in an interview. "I said, 'Can I go back?' and they said, 'You are in Mexico now, and you are under arrest.' "

Johnston was handcuffed and charged with possession of illegal weapons and attempting to smuggle them into Mexico. He spent the next 13 days in a Mexican jail while the U.S. military, State Department and California congressmen wrangled with the Mexican government and its judicial system to get him out. He was freed last Friday.

On one level, the incident illustrated the tough weapons laws Mexico has embraced to combat drug violence, crime and simmering rebel insurgencies. But in a broader sense, Johnston's jailing also exposed the ambivalence in U.S.-Mexican relations, particularly along the border. Despite strong family and cultural ties across the 2,000-mile line and unprecedented government and business cooperation, the neighbors are as separated by mistrust, fear, anger and resentment as ever.

"Mexico is very hypersensitive about its national sovereignty--their attitude is that we liberated half of Mexico" by annexing parts of Texas and California in 1848, said Rep. Brian Bilbray (R-Calif.), one of the leaders in the effort to win Johnston's release.

Since 1998, for instance, 302 Americans have been arrested on weapons charges in Mexico and 68 are still incarcerated here, according to the U.S. Embassy in Mexico City. Bilbray said that other recent occurrences--particularly fees for U.S. tourists going to Mexico and bonds of up to $800 for U.S. vehicles driven into the country--are signs that some Mexicans "are looking for a hassle."

The view from south of the border is not much different. Mexicans often are angered by what they see as America's arrogant treatment. And while some Americans have a laundry list of complaints about Mexico, Mexicans have their list, too.

It includes "meddling" by U.S. human rights groups and citizens in the leftist uprising in the southern state of Chiapas; executions of Mexicans in U.S. criminal cases (the death penalty is unconstitutional in Mexico); and nuclear waste dumping in facilities along the Texas-Mexico border.

Topping the list, however, are U.S. attitudes toward immigration. Many Mexicans say immigrants fill a vital need for cheap labor in the United States. And they say the value Mexican immigrants add to U.S. products and the taxes they pay more than offset the cost of the services they use. Many labor experts agree.

Yet about 150,000 Mexicans were expelled from the United States last year, and hundreds die in car accidents and from drowning, exposure and dehydration while trying to illegally enter the United States.

"There is a sense of partnership that prevails as you move away from the border, but if you are on the border, you are in a war zone," said Mexican Sen. Adolfo Aguilar Zinser, who frequently meets with his U.S. counterparts on bilateral issues. "So if a U.S. Marine crosses the border with a weapon, it's assumed to be an aggression. And if a Mexican approaches the border, he's assumed to be trying to illegally cross."

Drugs continue to be a flash point as well, and tough weapons laws are a key part of the Mexican government's effort to crack down on the drug scourge. In recent years, dozens of Mexican police working in anti-drug units have been killed in clashes with drug dealers. As a result, Mexico has adopted laws mandating prison terms of up to 12 years for people possessing rifles larger than .22-caliber, most handguns and ammunition.

The laws have ensnared numerous U.S. citizens who have strayed into Mexico on hunting trips or who did not know about the restrictions. Often, citizens see signs posted along the highway warning that taking guns into Mexico is prohibited, but only when they have passed the last U.S. exit and cannot turn around.

Last year, five young men from Louisiana who were dove hunting in the Texas border region were arrested and imprisoned for 33 days after entering Mexico with six shotguns. A 60-year-old Texan spent seven months in a Mexican jail when police found in his car a handful of ammunition that was left over from a recent gun show.

And two weeks ago, an agent for the U.S. Border Patrol was held for seven hours in Mexico on a weapons violation after he fell across the border while wrestling with a drug suspect on the main bridge between El Paso and Ciudad Juarez. The drug suspect--who ran from his car after it was stopped by U.S. Customs inspectors--was not taken into custody by Mexican police, U.S. officials said. They said they found 132 pounds of marijuana in his car.

Johnston, the marine sergeant stationed at Camp Pendelton 70 miles north of the border, said he had never been to Mexico before, and when questioned by Mexican border police when he arrived early in the morning of Oct. 30 showed them his two unloaded and properly licensed guns--a 9mm Beretta pistol and a Colt target rifle--in a bag locked in a tool chest in the bed of his 1996 Chevrolet pickup. He said he was carrying them because he intended to shoot at a practice range after his shift.

Johnston said he drove his own vehicle to the border, rather than ordering one from the Camp Pendelton motor pool, because he wanted to save time. Marine officials said that is a standard and acceptable practice. The truck and weapons are still in Mexican custody.

"I realize what I did broke the law, but I saw the sign saying no weapons and the next thing I knew I was 20 feet inside Mexico being arrested," he said. "I had nothing to hide, so when they asked if I had any weapons, I showed them to them."

Juan Rebolledo, the top bilateral affairs official in Mexico's Foreign Ministry, said the country has tried to make enforcement of its weapons laws more reasonable at the border, hoping to distinguish between weapons runners and law-abiding people who accidentally cross with arms. Johnston's case, he said, moved into the courts before high-level government intervention helped resolve it.

"We have very, very strict laws, and he should have known those weapons couldn't be brought with him into Mexico," Rebolledo said. "But we should have known about the case sooner. Once it got to a judge, it became a lot more complicated."