Once, the barren mesas and shrub-covered canyons that extend east of the Pacific Ocean held the most popular routes for illegal immigrants heading into the U.S. Dozens at a time sprinted to waiting cars or a trolley stop in San Diego, passing border agents who were too busy herding others to give pause.

Now, 20 years after that onslaught, crossing would mean scaling two fences (one topped with coiled razor wire), passing a phalanx of agents and eluding cameras positioned to capture every incursion.

The difference, Homeland Security Secretary Janet Napolitano said on a recent tour, is like “a rocket ship and a horse and buggy.”

In pure numbers it is this: Where border agents made some 530,000 arrests in San Diego in fiscal year 1993, they had fewer than 30,000 in 2012.

There is no simple yardstick to measure border security. And yet, as the debate on immigration reform ramps back up, many will try.

“Secure the border first” has become not just a popular mantra whenever talk turns to reform but a litmus test for many upon which a broader overhaul is contingent.

“We need a responsible, permanent solution” to illegal immigration, U.S. Sen. Marco Rubio, the Florida Republican who is working to develop a reform plan, said in his State of the Union response this month. “But first,” he added, “we must follow through on the broken promises of the past to secure our borders and enforce our laws.”

In fact, the 1,954-mile border with Mexico is more difficult to breach than ever. San Diego is but one example.

Two decades ago, fewer than 4,000 Border Patrol agents manned the entire Southwest border. Today there are 18,500. Some 651 miles of fence have been built, most of that since 2005.

Apprehensions, meantime, have plummeted to levels not seen since the early 1970s — with 356,873 in FY2012. Compare that to 1.2 million apprehensions in 1993, when new strategies began bringing officers and technology to border communities in California, Texas, New Mexico and Arizona. Now sensors have been planted, cameras erected, and drones monitor the borderlands from above.

But for those who live and work in communities along the international boundary, “secure” means different things. In Arizona, ranchers scoff at the idea. In New Mexico, locals worry about what’s heading south in addition to flowing north. And in Texas, residents firmly believe that reform itself would finally help steady the flow of people and drugs.

These places have been transformed. Sealed? No. But as one border mayor asked: “How secure is secure?”

San Diego

Don McDermott spent most of his 21 years in the Border Patrol working the San Diego sector. He remembers the “banzai runs,” when hordes of immigrants would storm inspection booths at one international crossing, scattering as they ran past startled motorists.

Back then, migrants crossed with audacity — even played soccer on U.S. soil as vendors sold tamales.

The tide turned when the U.S. government launched “Operation Gatekeeper” in 1994, modeled on a crackdown the previous year in El Paso, Texas. The effort brought 1,000 additional agents to San Diego. They parked their trucks against a rusting 8-foot-high fence made of Army surplus landing mats, and refused to yield an inch. They called it “marking the X.”

More than manpower helped to shut down the path into San Diego. An 18-foot-high steel mesh fence extending roughly 14 miles from the Pacific Ocean was completed in 2009, with razor wire topping about half of it. A dirt road traversing an area known as “Smugglers Gulch,” which border agents had to navigate slowly, was transformed into a flatter, all-weather artery at a cost of $57 million.

This past year the Border Patrol’s San Diego sector, which covers 60 miles of land border, made fewer arrests than in any year since 1968. Agents averaged 11 arrests each, a change that marvels veterans. Agents today may even pursue just one crosser during several shifts.

Nogales, Ariz.

In nation’s busiest illegal corridor, ranchers scoff at “secure.”

The question of border security hits close to home to those who work the land in southern Arizona. It was here, in 2010, that cattle rancher Robert Krentz was gunned down while checking water lines on his property near Douglas. Local authorities said they believe the killer was involved in smuggling either humans or drugs.

Defining “secure border” in Arizona is never easy. Just last week, U.S. Sen. John McCain hosted two town hall meetings on immigration reform in his home state, and was left defending a plan he’s been developing.

The crackdowns in Texas and California in the 1990s turned Arizona’s border into the busiest for human smuggling for 15 years running now.

In 2000, agents in the Tucson sector made more than 616,000 apprehensions — a near all-time high for any Border Patrol sector. The number eventually began dipping as the agency hired more than 1,000 new agents and the economy collapsed. State crackdowns such as the “show me your papers” law — requiring police enforcing other laws to question the immigration status of those they suspect are in the country illegally — are also thought to have driven migrants away.

The result: the sector had 120,000 apprehensions in fiscal 2012.

But the amount of drugs seized in Arizona has soared at the same time. Agents confiscated more than 1 million pounds of marijuana in the Tucson sector last year, more than double the amount seized in 2005.

In Nogales, Sheriff Tony Estrada has a unique perspective on both border security and more comprehensive immigration reform. Born in Nogales, Mexico, Estrada grew up in Nogales, Ariz., after migrating to the U.S. with his parents. He has served as a lawman in the community since 1966.

He blames border security issues not only on the cartels but on the American demand for drugs. Until that wanes, he said, nothing will change.

“I say the border is as safe and secure as it can be, but I think people are asking for us to seal the border, and that’s unrealistic,” he said.

Asked why, he said simply: “That’s the nature of the border.”

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