The idea was to restore the rule of law, to bring order to a chaotic situation. The results became more and more apparent. Crime rates went down in the border region. Today, the number of migrants crossing is at a 30-year low. That's because of years of bipartisan work on this issue.

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

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.

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.

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.

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.