A new law might hobble the US-Mexican fight against drug cartels

MEXICO CAN be relieved to finish the Trump era in one piece. Swingeing tariffs and border closures, ever threatened, never came. Nor did the bill for a wall on America’s southern border which Donald Trump promised to build at Mexico’s expense. The president’s attack on the North American Free Trade Agreement, which includes Canada, catalysed advocates of trade and integration. One could almost conclude that Mr Trump will leave relations between the two countries sturdier than he found them.

But the Trump show always has a twist. On October 15th Salvador Cienfuegos, Mexico’s defence minister from 2012 to 2018, was arrested in California on drug-trafficking charges, which he denies. Mexico was furious to learn that the United States had investigated him in secret for years.

On November 18th, in a shocking reversal, the United States dropped the charges against Mr Cienfuegos, who returned swiftly to Mexico. Court documents cite “sensitive and important foreign policy considerations”, implying that Mexico threatened to halt cross-border security co-operation if Mr Cienfuegos did not go free. Prosecutors in the United States cannot recall any other occasion where a foreign government has appeared to snatch its own citizen from the jaws of America’s criminal-justice system so brazenly.

The episode, described by one...