Walls and barriers alone are insufficient to insure security.

We should be wary, particularly with our closest friends and allies, of breaking down the trust and confidence that lie at the foundation of relationships.

As former Homeland Security Secretary Janet Napolitano pointed out, if you build a 50-foot wall, you'll soon be confronted with a 51-foot ladder. You need a strategy that involves layered defense: deployed patrols, sophisticated sensor equipment, and surveillance from the air. That is what has had a positive impact over the last generation.

Mexico has become a robust democracy with a robust press and an active legislature. It has gone from being a sending country for migrants to a transit country, and increasingly a receiving country for migrants in its own right.

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.

We have to remember that information sharing is restricted by legal barriers and cultural barriers and by the notion that information is power and therefore should be hoarded so if you share information you can extract something in exchange. In today's digital online world, those who don't share information will be isolated and left behind. We need the data of other countries to connect the dots.

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.