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.
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.
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 in recent years, their lowest levels 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 Barack 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.