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.

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.

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.

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

Every air traveler entering Mexico is vetted against US databases. The air passenger screening system Mexico has in place involves these checks against US national security and criminal data bases. There are plainclothes US officers stationed at airports in Mexico working with Mexican immigration officials to protect the United States. This joint security program has been in place for at least six years and is a huge asset.