Two young girls watch the World Cup on television in at a federal holding facility in Nogales, Ariz. on June 18, 2014. (AP/Ross D. Franklin, Pool, File)

A year ago last summer, an influx of children migrating across the Mexican border created a crisis for the Obama administration. Now children are showing up at the border again -- some of them with their parents, but many traveling on their own to escape violence in Central America.

And it's not a problem Donald Trump's wall -- or any of the other heavy-on-security proposals -- are likely to solve.

These immigrants -- unlike immigrants who have crossed illegally in the past -- are not dodging Border Patrol agents by tunneling under fences or making a dangerous journey on foot through uninhabited desert. The migrants have special legal status because they are not just here to work, but are fleeing turmoil at home, and they're actually seeking out men and women in uniform.

"They're surrendering to the first Border Patrol agent they can find," said Marc Rosenblum, deputy director of the Migration Policy Institute's program on U.S. immigration.

It's a serious issue, and not one that's likely to go away, experts on immigration argue. For the most part, though, politicians aren't putting forward proposals to address it. Instead, they're focused on security -- a separate question from that of the migrants turning themselves in.

On Tuesday, Trump renewed his promise to build a wall along the southern border. And Texas Gov. Greg Abbott ordered the National Guard to remain on the border in force.

"It doesn't matter how many more Border Patrol agents we put on the border. That's not going to have an impact here," Rosenblum said.

About 10,500 children crossed the border alone in October and November, according to the institute's analysis of federal data. Like the children and families from Central America who came en masse a year ago last summer, causing a crisis for an unprepared Obama administration, those crossing the Rio Grande this fall represent a change in the kind of immigrant that federal agents are likely to encounter.

It is a shift that will require political leaders to expand the debate beyond border security, experts say.

For many years, most of the migrants crossing the border were coming to the United States to work. Many of them were young Mexican men who had left their families behind. They crossed illegally, hoping not to get caught.

Today, though, the influx of Mexican migrants has largely ceased. The number of undocumented Mexican immigrants living in the United States has been declining for eight years, according to the Pew Research Center. More Mexican immigrants have been traveling south across the border, out of the United States, than have been coming north and entering the country.

The decline in their numbers has a few causes, according to Pew.

For one, the U.S. economy suffered a profound financial crisis in 2008, but the Mexican economy continued to improve, giving people there more opportunities to work. Aside from the financial collapse, Mexican mothers have been bearing fewer children as the country's economy has developed, resulting in a smaller group of young people seeking employment.

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These days, though, immigrants aren't just coming from Mexico. They're coming from the increasingly chaotic Central American nations of Guatemala, El Salvador and Honduras. And they're not young men traveling alone. Instead, many of them are mothers coming with their children, or children coming without their parents. They're not primarily looking for work, but rather fleeing violence at home.

Rosenblum estimated that about 38 percent of immigrants apprehended at the border last year were women and children, compared to 18 percent in 2010. Fewer than half were from Mexico, compared to an average of more than 95 percent over the past half-century or so.

U.S. immigration law provides no special protection to people who are coming to this country for purely economic reasons. Because this new group of immigrants is leaving bloodshed behind, they can make a claim to asylum. If they receive that status, they are allowed to stay here legally.

These immigrants have nothing to fear from the courts. Instead, they hope to have an asylum official interview them and give them permission to stay pending a hearing before a judge. Detention is their goal.

"No amount of border enforcement" will solve this problem, said Pia Orrenius, an economist at the Federal Reserve Bank of Dallas who studies immigration.

Instead, she said, the question confronting policymakers is providing immigration courts with enough manpower to handle the increasing numbers of immigrants, admitting those who have a legitimate claim to asylum and deporting those seeking to exploit the system.

"The whole rhetoric around migration has to change," Orrenius added.

To be sure, the candidates are aware of the problem. A document from Trump's campaign includes a paragraph on asylum, recommending changing the standards so that more claims are denied. Sen. Ted Cruz (R-Tex.) calls for the summary dismissal of baseless claims and the immediate deportation of those who surrender to border enforcement in his proposal. Many immigrants do not appear for subsequent hearings after their initial interview, instead remaining in the United States illegally.

The details of these proposals matter, though. Rosenblum noted that the United States has a long tradition of accepting those are fleeing strife at home, and many of the migrants do have what the courts would traditionally regarded as legitimate complaints of persecution by violent gangs in Central America.

Rosenblum has calculated that of the migrant children whose cases do eventually go before a judge, about 83 percent are granted relief of some kind if they have a lawyer. (Foreigners have no right to representation in immigration courts. The majority of children do not have an attorney, and almost all in that group are ordered deported.)

Likewise, requiring all employers to electronically verify their employees' eligibility to work in the United States -- as several Republican candidates have advocated -- might do little to deter women and children who are not coming here for employment but for safety. Many might hope to rely on friends or family here already for support once they arrive.

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For the most part, the candidates have not broached these difficult questions on the campaign trail. They're focused instead about border security, using a set of talking points that Americans politicians have been repeating for decades.