From where Mr. Miller sits, illegal immigration is driven by a belief among people outside the United States that those who make it across the border will be allowed to stay indefinitely. “The success rate is the predominant factor that drives illegal migration,” he told us. By instituting a zero-tolerance policy, Mr. Miller said, the administration is sending a message that should reduce the flow of illegal immigration into the country. He conceded that the policy has not reduced the immigration numbers, but said, “It’ll take a few months of sustained effort.”

2. Anything less than zero tolerance at the border creates what Mr. Miller calls “perverse” incentives for lawbreakers.

Mr. Miller used the example of speeding laws in the United States. Imagine, he said, if the police decided that speeding laws didn’t apply to people who have a child in the back seat. “Could you imagine what the consequences of that would be? Well, one thing, a lot more child endangerment,” he said, comparing the situation to the lack of enforcement of illegal border crossings under prior administrations.

3. Continuing policies in place during the Obama administration would cause what Mr. Miller called a “vicious cycle” that would increase illegal immigration.

Mr. Miller said a “giant hemispheric shift in migration patterns” was driving people toward the United States, which he said must respond with tighter borders. He said Obama-era policies would allow the number of immigrants to “spiral upward endlessly.” Using a favorite phrase, Mr. Miller said: “So you have to turn the ship. And so again, it’s a whole-of-government approach.”