They followed him back to his mountain retreat, pointed guns at his head and at those of the Eritrean migrants he had just picked up, and jailed him. The migrants were hustled back to Italy.

After 48 hours, the prosecutor in Nice decided not to pursue charges, having concluded that Mr. Herrou was acting for humanitarian reasons, his lawyer said.

In another demonstration of France’s jumbled approach to migrants, the police know exactly where Mr. Herrou is and what he is doing. Yet they mostly leave him alone.

In Breil-sur-Roya, an old French-Italian village of ocher houses in the valley by a quiet lake, Mr. Herrou is something of a celebrity. At the Friday night local council meeting, townspeople clapped him on the back, greeting him warmly. That afternoon he had shared a beer with the town’s Socialist mayor in the main square.

“Yes, of course, we know,” the mayor, André Ipert, said in an interview. “Yes, of course, he is outside the law. This happens in France.”

That very day, three Sudanese migrants had straggled into Breil’s tiny town hall. The mayor did not turn them over to the police.

Others agreed with the assessment, and have done the same.

“We think we are doing what we should do, as citizens,” said Françoise Cotta, a well-known Paris lawyer who lives part time in Breil. She is part of the smugglers’ network. “Down there I am a citizen, and what I do is illegal,” she said. “And I help them.”

In fact, the migrants’ odds are vastly improved if they have the good fortune to stumble on Mr. Herrou and his allies.

In the town of Menton, a scene plays out daily that is a counterpoint to all of the efforts of Mr. Herrou, whose farm is just 20 miles away.

It went like this on a recent Sunday evening: The 6:16 from Ventimiglia glided into the tidy suburban station of Menton-Garavan, the first on the French side of the border.

Immediately, French riot police officers took up positions on the platform They boarded the little suburban train and found what they were looking for: three African teenagers trying to sneak into France from Italy.

The ragged boys were ordered off, marched down the platform and commanded to empty their battered backpacks, while smartly dressed passengers averted their gazes. Soon the return train to Italy arrived, and the boys were put on it.

The officers say they don’t like rounding up the migrants and pulling them off trains. “These are just minors, totally helpless,” one said, grimacing.

Many train conductors don’t like it, either, but neither do they protest. “There are women and children,” the conductor, watching the operation, muttered. “It’s horrible.”

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