The border barriers rise out of the Pacific Ocean, climb craggy California peaks, streak across Arizona desert valleys and meander through cattle ranches and fields of sorghum and citrus in South Texas.
Tall steel fencing separates border communities. Camera towers and bright rows of stadium lights aim at smugglers' enclaves in Mexico. Migrants seeking out traditional crossing routes find them blocked, and many give up.
But migrants still get across, by seeking out the one road or one mountain range or one desert trail beyond the reach of the U.S. Border Patrol.
Photos: Securing the border with Mexico
Scenes like these form the bedrock of the immigration reform debate in Washington. The Obama administration backs a pathway to citizenship for more than 11 million illegal immigrants, and a bipartisan group of U.S. senators is working on legislation to make it happen. But first, the eight senators have agreed, the border must be certified as secure.
Memories are still fresh of the reforms of 1986, which led to citizenship for 3 million undocumented immigrants but did not strengthen the Mexican border. Millions of migrants poured through in the ensuing years, making a mockery of claims that the immigration problem had been solved.
Failure now to agree on the definition of a secure border, and on how much money to spend to achieve it, would probably kill the current reform movement.
Photos: Securing the border with Mexico
Obama administration officials claim the frontier is more secure than ever, benefiting from the billions of taxpayer dollars spent on border defenses. There are 18,500 U.S. Border Patrol agents on the U.S.-Mexico border now, compared with 3,222 in 1986. Barriers have been built along nearly 700 miles. In 1986, most of the frontier was wide open.
Arrests of migrants have hovered around 350,000 per year recently, the lowest level since the 1970s. Falling crime rates in border communities make them some of the safest in the country. Authorities have regained control of once-trampled areas, opening the way for new subdivisions, shopping centers and industrial parks.
"We have gone above and beyond what was requested by the very Republicans who said they supported broader reform as long as we got serious about enforcement," said President Obama during a trip to the border at El Paso in 2011. "All the stuff they asked for, we've done."
But critics consider some recent gains illusory. Migrant flows have receded overall because of the lack of jobs in the U.S. The true test will come when the economy improves, they say.
Some current and former border officials doubt defenses will hold completely, especially because certain areas remain porous. They say more enforcement measures are needed. One of the critics' oft-cited facts: There are fewer agents patrolling a nearly 2,000-mile border than there are cops, about 34,500, in New York City.
Border security is still a work in progress, said Ronald Colburn, a former chief U.S. Border Patrol agent. "We're not there yet. The border is still a high-risk situation," said Colburn, who oversaw some of the fence construction. Migrant arrests, which topped 120,000 in Arizona alone last year, he said, remain at worrisome levels.
Homeland Security officials express frustration at what they consider unrealistic expectations. Because accurate measurement of migrant traffic has always been elusive (how can migrants be counted if they're not caught?), security levels will always be open to interpretation.
The standards demanded by some lawmakers are unattainable, no matter the investment. "Even the heaviest concentration of fencing, all-weather roads, 24-hour lighting, surveillance systems and Border Patrol agents cannot seal the border completely," U.S. Border Patrol Chief Michael Fisher recently told Congress.
Down along the endlessly varied terrain and ever-shifting flows of migrant traffic across the 1,954-mile U.S.-Mexico border, the picture is decidedly mixed, giving ammunition to both sides to make their arguments.
When the federal government moved to control the border in the 1990s, it focused on America's frontier towns and cities. In enforcement parlance, such communities are called melt zones, places where homes and shops lie but a quick sprint away from Mexico. Migrants who breach the border can slip amid the bustle of pedestrians and traffic, melting into the U.S.
Those who argue that great strides have been made in securing the border point to the melt zone cities as evidence.
The melt zone in the California town of Calexico lies across 1st Street from the Baja California city of Mexicali. In 1999, border authorities erected a 15-foot steel barrier dividing the cities. It wasn't intended to stop migrants, just slow them enough for agents to respond.