With the world focused on the refugee crisis in Europe, it's easy to forget the ongoing migration crisis on the U.S.-Mexico border. Thousands cross that border each year and hundreds die trying. In the run-up to the U.S. presidential election, it's a hot topic. No U.S. government has deported more people than that of Barack Obama. But that doesn't mean Republicans are satisfied. Republican presidential frontrunner, Donald Trump, has called for mass deportations, saying, "get them the hell out of our country."

It's fuel for punchy political speeches but the daily realities for millions of undocumented immigrants living in the United States are often complex. Life Links' #borderlines episode explores one story of a deportee that's not easy to dismiss: A former U.S. Marine named Daniel, who grew up in America and fought for America, was later deported to Mexico. Daniel's case may seem extreme, but he's just one of hundreds of thousands who are forced to leave the country they call home every year.

Political scientist and border expert David Shirk from the University of San Diego has been closely following the U.S.-Mexico border issues. Life Links reporter Gönna Ketels talked to him about immigration policy and the situation of deported veterans.

Why is the United States' southern border so highly secured today?

We estimate that about 200,000 to 400,000 people come into the United States without authorization in any given year. Half of those are people who come here on a visa and overstay their visa, but at least 40 percent are individuals who cross the border without authorization. As a result of that migration, the United States has been constructing walls and increasing border security since the 1980s to try to prevent undocumented immigration literally by building physical barriers and adding manpower.

That hasn't been particularly effective – people have come into the United States in tunnels, they travel through the far extremes of the desert and they come in through boats or other means.

At this point, there are somewhere between 11 and 12 million undocumented persons living in the United States. Many of these, perhaps 3 or 4 million, came into the United States as small children. In some cases, they didn’t even know that they arrived without proper documentation and that they weren’t U.S. citizens. They now find themselves living in a country that they know and love, but realizing that they’re actually citizens of somewhere else.

A law passed in 1996 made it much easier to deport permanent residents from the U.S. Since then, we have seen the largest wave of deportations from the United States in our history. Over the course of the Obama Administration, the United States Government has dramatically increased the number of deportations. We have seen over two million deportations, just in the last six or seven years.

I was shocked at the number of migrant deaths of people attempting to cross the border: 307 in 2014. Could you argue that this is one of the deadliest borders in the world?

Europe has us beat on that but it’s true. It's something we've known for a long time in the border region but it’s almost never talked about in Washington D.C. There has been more coverage of the European migrant deaths in the U.S. media than there has ever has been of Mexican and Central American migrant deaths crossing into the United States.

Up until about 1992 there were very few physical barriers to entering the U.S. Mexico border and there was relatively little enforcement. Over the course of the 1990s, we saw the development of much more intense and concentrated border enforcement. This drove undocumented border crossers out to the deserts and mountain regions of southern California where it’s much more dangerous to cross.

The U.S. Government did this knowingly. They knew that it would put more people at risk, but they wanted to create order at the border. After this, the number of people dying in the deserts and mountains went up from a few dozens into the hundreds. By the late 1990s, we would have 300 to 400 people dying every year trying to come into this country. That has more or less continued: There are 200 to 400 people dying every year. At this point, more people have died crossing the U.S. Mexico border than on 9/11, in the terrorist attacks.

You mentioned that you don’t find border enforcement here very effective. In Europe, we are seeing new security borders being put up right now. Would you say that’s the right way?

Physical border security is not the answer to unwanted immigration. The answer is to promote development in the countries that people are coming from. People primarily leave their home countries because there are no decent opportunities for work in those communities.

Economics drives the issue. That would be the ultimate solution to this problem, to spread development all over the world, so people don’t have to leave their home communities. The persistent problem over the course of the era of globalization has been that we have free-flowing goods, free-flowing capital, but not free-flowing opportunities for workers and those workers seek jobs elsewhere.

Barack Obama has promised immigration reform and the naturalization of undocumented migrants. How does that fit with a record high in deportations?

President Obama and the Democrats seem to want to ramp up deportations because it sends a message that they are tough on immigration, so that they can negotiate with Republicans who claim the Democrats are soft on the issue.

One of the challenges for Barack Obama is that on one hand, he needs to seem tough on immigration, on the other hand, he’s trying to win concessions for immigration reform. The Obama Administration sees these two things going hand in hand: Obama has to deport people, in order to save people. That’s a very difficult balance to maintain. So the Obama Administration has tried to focus deportations on the targeting of so-called "criminal aliens".

In the case of Daniel, the young veteran we met in Mexico, his offence was being an undocumented migrant in the U.S. and lying about his status when he joined the Marines. When the military found out he’d already served four years - including action in Iraq - he had to leave the country. That seems rather harsh. How are these decisions made?

The reality is that there is very little accountability for our immigration enforcement system. The Immigration and Customs Enforcement Agency has broad latitude and an ability to hold individuals essentially without habeas corpus, without rights. This is partly because the U.S. Constitution is structured only to establish rights for citizens. As foreigners, individuals are not protected by the Constitution. You’re not subject to the Bill of Rights, you don’t have freedom of speech when you are in the country as a foreigner. Right now, we’re living in a time when we have granted our enforcement agencies excessive powers and we have waived the rights of individuals who are subject to our immigration system.

How is it even possible that foreigners who served in the U.S. armed forces, who fought in wars, are deported from the United States?

We don’t know exactly how many people serve in the military who are not U.S. citizens, but the estimate is somewhere around 20,000 or 30,000 in the past few years.

The reason many veterans are getting caught up is because they have stepped over the line, they have committed some kind of misdemeanor or felony that puts them on the radar of immigration enforcement. Unfortunately, if you’re a veteran of a foreign war, if you have post-traumatic stress disorder, the probability that you’re going to use drugs or have a disorderly conduct violation on your record goes up. Many of them wind up getting deported because they are committing crimes or using drugs precisely because they are veterans.

Once they are on the Mexican side of the border, in most cases, they have no direct personal connection to Mexico because they have lived most of their life in the United States. They often have very few transferable skills, many of them don’t even speak Spanish fluently. So their opportunities for living in Mexico are very limited.

What are the chances of deported veterans to fight that decision and to come back to the United States?

Once you’ve been deported, that’s it. We have a no-return policy for deportees. If you do come back, it aggravates your status and you will surely be deported again.

In Obama’s proposed immigration reform there is no mention of deported veterans. Do you think there is a chance that their situation might change in future?

Unfortunately, the interests of the 20,000 or 30,000 veterans who have been deported to other countries have not made it onto the agenda. Part of the reason is, it’s a very small niche group. But it’s a no-brainer: The highest service that one can offer to a country is military service and when you look at our population, the vast majority of us don’t do that.