In recent weeks, the southern U.S. border has developed into a front line of sorts. Almost daily, strident demonstrators protesting the arrival of thousands of undocumented immigrant children pick verbal fights with their opponents — who answer in kind.

At issue: the influx of tens of thousands of unaccompanied minors from Central America.

On Tuesday, President Barack Obama requested $3.7 billion in emergency funds to deal with the influx of Central American children.

The money would mostly be spent on detention, immigration courts, prosecution of smuggling networks and care and transportation of the children, who often travel without their parents, the White House said.

Heather Piña Ledezma, 6, attends a rally with her mother who is from Mexico on the steps of the St. John's Episcopal Church near the White House, July 7, 2014. Image: Tom Williams/CQ Roll Call/Getty Images

Obama, who has called the border situation a "humanitarian crisis," will meet with faith leaders and local officials in Texas to discuss it on Wednesday.

But what's behind this crisis? Here are the things you need to know:

Tens of thousands of children are coming from Honduras, El Salvador and Guatemala. Though many children had already begun arriving by late 2011, the wave of underage immigration has surged this year. The number of children and teenagers crossing the border from Guatemala, Honduras and El Salvador alone is expected to reach as many as 90,000 by the end of 2014, according to the Los Angeles Times.

The children are fleeing poverty and atrocious gang violence in their home countries. A little-known anti-trafficking law signed by President George W. Bush has also drawn a lot of children, as The New York Times reported. (The legislation gave significant protections to children entering the country alone, including an immigration hearing and consultation with an advocate.)

Meanwhile, critics of Obama's immigration policy blame his decision in 2012 to allow some immigrants who were illegally brought into the country as children to stay and work without fear of deportation. These critics say the policy has been misunderstood by people in Central America as an incentive to send their children to the United States.

Sometimes, as the pictures below reveal, the children are held in overcrowded and unsanitary conditions. A number of images obtained by Breitbart Texas, including the one below, show holding cells at nine different U.S. Customs and Border Protection facilities in southern Texas.

Image: Breitbart Texas/Brandon Darby

Though Obama has repeatedly declared himself in favor of immigration reform, he has deported more than 2 million people — more than any other president in history — and often for minor infractions.

In fact, the rate of immigrants being apprehended at the border is near a historic low, in part as a result of the recent economic crisis as well as aggressive deportation by the Obama administration. Still, the vocal and sometimes contentious protests in recent weeks continue.