TRUMP: "These (border) agents will tell you where walls go up, illegal crossings go way, way down ... San Diego used to have the most illegal border crossings in our country. In response, a strong security wall was put in place. This powerful barrier almost completely ended illegal crossings ... Simply put, walls work and walls save lives."
THE FACTS: It's a lot more complicated than that.
Yes, Border Patrol arrests in the San Diego sector plummeted 96 percent from nearly 630,000 in 1986 to barely 26,000 in 2017, a period during which walls were built. But the crackdown pushed illegal crossings to less-patrolled and more remote Arizona deserts, where thousands died in the heat. Arrests in Tucson in 2000 nearly matched San Diego's peak.
Critics say the "water-balloon effect" — build a wall in one spot and migrants will find an opening elsewhere — undermines Trump's argument, though proponents say it only demonstrates that walls should be extended.
The Government Accountability Office reported in 2017 that the U.S. has not developed metrics that demonstrate how barriers have contributed to border security.
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EDITOR'S NOTE _ A look at the veracity of claims by political figures