The image and the costs of a Berlin-like wall or a Great Wall of China is something that the American people have not accepted to date.

Homeland security is inherently transnational today. There's hardly anything adverse that happens in our homeland that doesn't have a cause or effect that's generated abroad. Increasingly, we must rely on our allies and foreign governments to share information and data to secure our country.

Our security in a global world must be looked at on a continental basis.

The Mexican people are increasingly middle class, and Mexico has substantially become a middle-class society. This is true despite the significant poverty, and the class and geographic inequality that have deep historical roots.

In the last generation we've moved past a U.S.-Mexico relationship that while friendly on the surface, and demilitarized for the most part, really was not a genuinely cooperative relationship. As a result of the U.S.-Mexico War in the 19th century, and the treaty of Guadalupe Hidalgo in 1848, half of what was Mexico was severed and became much of the western part of the United States. To add insult to injury, most Americans never knew that, and most Mexicans have never forgotten it.

There have been enormous advances since 9/11 to build a very robust set of targeting procedures and watch lists to screen travelers coming to the United States. The National Targeting Center in Virginia run by Customs and Border Protection checks the background of every traveler who seeks to enter the country.

I think there's no question that the barriers, the fences and in certain urban areas, the walls, have had an important effect in terms of increasing the manageability and the security of the border. But in fact as Secretary of Homeland Security General John Kelly acknowledged at his confirmation hearing, walls and barriers alone are insufficient to insure security.