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.

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.

The work the Mexicans are doing in terms of migration control on Mexico's southern border is crucial to our own border security.

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.

The nationalism and the protectionism that was built into the Mexican Revolution in 1910 and that characterized the Mexican attitude to the United States for much of the 20th century were difficult to overcome. But that actually has occurred. And the cooperation, trust and confidence that have been built is not something that should be abandoned without great consideration for the potentially grave consequences to the United States.

If you build a 50-foot wall, you'll soon be confronted with a 51-foot ladder.

We can't defend the country by looking at the borderline as the first line of defense rather than as the last line of defense. We have to secure the flow of goods and people by engaging with foreign entities. We assure our security by securing the flows as early as we can before they arrive and as far away from our borders as we can.