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BOTH the United States and Mexico have approached the war on Mexican drug cartels with Colombia in mind.

Washington’s Merida Initiative, loosely modeled on its Plan Colombia antidrug campaign from a decade ago, provides Mexico with money for helicopters, police training and command-and-control technology. The Mexican government, meanwhile, has taken steps to modernize its judicial system, purge the police of corruption and improve intelligence services.

But according to a Government Accountability Office report released this summer, the billions of dollars spent by Mexico and the United States over the last four years have done little to thwart Mexico’s cartels.

The problem is that the two countries have ignored a fundamental lesson from the Colombian...