It is unclear whether the farmer knew the significance of the stone, which has 1819 carved into its face.
In theory, moving the stone violates the 1820 treaty, said Mr. Chopin. “It’s very, very serious,” he said. “Well, ‘serious’ in quote marks because there are of course many more important things than this.”
Luckily, local officials in each country have seen the funny side of the situation.
“He made Belgium larger and France smaller; that’s not a good idea,” David Lavaux, of the Belgian district of Erquelinnes, said in an interview with the French broadcaster TF1. Mr. Lavaux is the village’s burgomaster, a position equivalent to mayor or chief magistrate.
Once he knew of the incident, Mr. Lavaux contacted Aurélie Welonek, who holds a similar position in the French village across the border. “I was happy as my town was larger, but the mayor of Bousignies-sur-Roc didn’t agree,” Mr. Lavaux said with a chuckle.
Mr. Lavaux said he would send the farmer, whom he did not identify, a formal letter demanding that the stone be returned to its rightful location. If the farmer does not comply, he could face criminal charges.
If no agreement can be reached, Mr. Lavaux said he would turn to Belgium’s foreign ministry, which would set up a Franco-Belgian commission to resolve the border dispute, a move that was last required in 1930.
Mr. Lavaux and Ms. Welonek said in interviews with French news outlets that they were convinced it wouldn’t come to that.
“We should be able to avoid a new border war,” Ms. Welonek told the newspaper La Voix du Nord.