Border Patrol commanders say they see no explanation for the drop-off across the entire 2,000-mile border other than stiffer enforcement deterring migrants. The slackening flow, they argue, belies the conventional wisdom that it is impossible to stem illegal migration. Many veteran officers in the force are now beginning to believe it can be controlled with enough resources.

The new measures range from simply putting more officers out on patrol to erecting stadium lights, secondary fences and barriers of thick steel poles to stop smugglers from racing across the desert in all-wheel-drive trucks. The Border Patrol has deployed hundreds of new guards to watch rivers, monitor surveillance cameras and guard fences.

In the Yuma headquarters of the Border Patrol, for instance, Chief Ronald Colburn said that with the help of the National Guard the patrol had doubled the agents in his sector to about 900, extended the primary steel wall eight miles past the end of the Mexican town of San Luis Río Colorado, and constructed a vehicle barrier six miles beyond that. “It’s the right mix, the right recipe,” he said.

The federal government has also begun punishing migrants with prison time from the first time they enter illegally in some areas. For instance, along the 210 miles of border covered by the Del Rio office of the Border Patrol, everyone caught crossing illegally is charged in federal court and, if convicted, sentenced to at least two weeks in prison.

That is an enormous break with past practice, when most Mexican migrants were simply taken back to the border and let go. People from Central American countries were given a court date and released on their own recognizance. Few ever showed up.

In San Luis Río Colorado, the effects of the stepped-up patrols are apparent. A year ago, migrants thronged the town park and cheap motels, where guides, known as “coyotes” or “polleros” offered their services. Now the park is nearly empty. The smugglers are telling their charges to take a bus to a spot called El Sahuaro about 50 miles east of town. From there the migrants make a dangerous two-day walk through rocky canyons and barren desert to reach Interstate 8.