MEXICO CITY — In the past, Mexico’s revolutions and internal wars have all been eruptions stemming from deep social problems. They unleashed enormous destructive power and took decades to run their course. But they were always followed by long periods of peace and economic development.

The country’s present social unrest has a different source and is of a different nature. If the sweeping economic reforms of 2013 attract investment and are implemented efficiently and honestly (two bigs ifs), the major remaining obstacles to real social progress will be the powerful force of organized crime and the weakness of legal and practical measures to stem it.

Since democracy came to Mexico in 2000, the country has sunk into a cycle of violence fed by intense criminality. Images circulating on social media starkly depict its horrific cruelty. It is true that narco cartels and other organized crime groups (with allies in high political positions) have grown vastly stronger since the 1970s. But no one foresaw the paradoxical cause of their huge expansion: the limits set by democracy on the formerly near-dictatorial power of the president.

The arrival of democracy has had a centrifugal effect in sharply strengthening local power. In places where local politicians and the police are corrupt, criminals have become autonomous and fearless. A kind of civil war with multiple fronts has developed — an intensifying conflict between the state and the cartels, as well as among the cartels....