How is the border defended now?
Through a combination of fences and patrols. The Department of Homeland Security has fenced off 651 miles of the 1,969-mile border between the U.S. and Mexico. The barriers are mostly near urban areas and international bridges; the U.S. Customs and Border Protection agency uses patrols to guard the more remote borderlands in California, Arizona, New Mexico, and Texas. The agency — currently larger than it's ever been, with over 58,000 personnel — spends $4 billion a year protecting the southwestern border with the help of 16,875 vehicles, 269 aircraft, 300 watercraft, and 300 camera towers. It even uses aerial drones to enhance the scrutiny. President Obama says that the U.S. has now "strengthened border security beyond what many believed was possible." Senate Republicans, though, say they won't agree to immigration reform until the border is better protected. "We must follow through on the broken promises of the past to secure our borders and enforce our laws," said Sen. Marco Rubio.

How many illegal entrants are caught?
In 2012, the Border Patrol apprehended about 357,000 people at the southwestern border — a 78 percent drop since 2000. The government points to the near record lows in illegal migration as evidence of its success at deterring would-be immigrants from crossing illegally. Critics, though, say the decline is mostly a temporary function of the shortage of U.S. job opportunities. In a recent report, the Government Accountability Office estimated that only 61 percent of those attempting to cross the border illegally are intercepted; fewer than 100,000 annually are thought to have settled here in recent years. The GAO found that just 44 percent of the border was under "operational control," with nearly two thirds of the remaining 56 percent "monitored," and the rest "low-level monitored." Critics say those numbers prove that security remains porous. "The bottom line is that we are far from having operational control of our borders," said House Homeland Security Committee Chairman Michael McCaul (R-Texas).

How can security be improved?
The Border Patrol could be deployed more efficiently. Currently, nearly 40 percent of the agency's staff is based in Tucson or El Paso, where intense enforcement has resulted in declining numbers of attempted crossings. San Diego has 80 Border Patrol pilots, while the entire Rio Grande Valley has just 15. To cover more-remote areas, Republican lawmakers such as Michele Bachmann and Ted Cruz have advocated erecting more fencing. Arizona has already committed to raising $50 million in private donations to completely fence off its 370-mile border with Mexico. "It's not complicated," said conservative commentator Charles Krauthammer. "Build the damn fence."

Why hasn't that been done?
The cost and logistical challenges would be immense. Estimates from Customs and Border Protection suggest a cost of more than $22.4 billion to build a fence along the entire southwestern border, which runs along riverbanks and through remote deserts, marshlands, and hill country. On top of that would come ongoing maintenance costs: In 2010 alone, the agency repaired 4,037 breaches in the existing fences. The government would also need to expropriate private land, which would be both expensive and unpopular. And the border would still need policing, as illegal immigrants will try to breach, tunnel under, or climb over any fence. It's also possible, some immigration experts say, that ramping up border security could have unintended consequences.

What would those be?
Many illegal immigrants currently cross the border for seasonal work, then go back over to their families in Mexico. Additional border security could halt that ebb and flow, causing many illegal immigrants to choose to stay in the U.S. in perpetuity. Advocates of immigration reform argue that since our economy creates a big demand for the low-wage agricultural, factory, restaurant, and service labor that Mexicans provide, the best way to prevent illegal entrants is to create a realistic system that allows people to come and go on temporary work visas. "It isn't possible to enforce our way out of our immigration problems," said Benjamin Johnson, executive director of the American Immigration Council.

What's likely to happen next?
For reform to move forward, Congress, the White House, and the American people have to agree on what is an acceptable standard of border security. Past precedents suggest this will be difficult. The American Immigration Lawyers Association recently noted that all of the proposed security benchmarks outlined in 2006, 2007, and 2010 immigration bills have been met or exceeded, yet some lawmakers insist that border security remains insufficient. To truly "close" the border, as some demand, would require the equivalent of a 1,969-mile Berlin Wall. "The only nations that have come close to such control were totalitarian, with leaders who had no qualms about imposing border control with shoot-to-kill orders," said political scientist Rey Koslowski of the State University of New York at Albany. As Republican and Democratic senators hammer out fresh immigration legislation, said El Paso Mayor John Cook, their first question should be a basic one: "How secure is secure?"

Paying to get across
Smuggling illegal aliens across the border is now a $6.6 billion industry for Mexican gangs. Rates range from $3,000 to $30,000 for transport across the border, and anyone who tries to cross without the paid assistance of the cartels that control various swaths of the border risks being beaten or shot. About 90 percent of illegal aliens now pay "coyotes" to sneak them across the border, according to the U.S. Immigration and Naturalization Service. As U.S. border security hardens, the gangs are devising ever more ingenious means of getting across. Smugglers have mocked up fake Halliburton trucks to cross through oil fields that straddle the Texas-Mexico border. They have also used stolen Fed-Ex, UPS, and AT&T service trucks to ferry illegal immigrants into the U.S.