Last week I visited Guatemala, where the new president-elect, Otto Pérez Molina, has promised to deploy the army to fight organized crime. Pérez Molina—himself a former army general—even said that he will follow Felipe Calderón’s lead in declaring an all out war against drug cartels. He should think twice about that strategy.

Let’s look at what happened to Mexico’s murder rate when Felipe Calderón came to power in December 2006 and launched a military offensive against drug cartels. The murder rate in that country, measured as the number of homicides per 100,000 inhabitants, had been experiencing a steady decline since the mid-nineties. However, it skyrocketed after the army went into the streets, unleashing unprecedented violence as the cartels fought back and escalated their vicious infighting.

Source: Global Study on Homicide 2011, United Nations Office on Drugs and Crime.

One clear phenomenon in Mexico is that, instead of placating violence, the deployment of the army helped to magnify it. The reason is that, even when the army could claim victory by killing or arresting a drug kingpin or dismembering a gang, it would only create a vacuum that other cartels would try to fill—violently. Mexico’s highly fractious criminal spectrum, with at least seven significant drug cartels vying for control of territory, is to some extent the result of the government’s war against organized crime.

Still, Mexico’s murder rate in 2010 (21.5 killings per 100,000 inhabitants) is about half that of Guatemala (41.4 murders per 100,000 inhabitants). There are two reasons why things could get much worse in Guatemala: First, the army is ill-prepared to fight the powerful Mexican cartels that already have a presence in that country. After the peace accord of 1996, the size of the Guatemalan army went from 50,000 troops to only 16,000. If the cartels have put up a fight to the better-equipped Mexican army, one can only wonder what would happen to its smaller and poorer Guatemalan equivalent. Second, even if the army is successful in weakening the cartels, the same vacuum phenomenon that takes place in Mexico would happen in Guatemala. So far, Mexico’s two most powerful cartels, Sinaloa and Los Zetas, control different parts of Guatemala’s territory, but they haven’t engaged each other in that country yet. That could change if the army strikes a significant blow to one of them, giving an opportunity to the rival.

Guatemalans elected Otto Pérez Molina for his promise to fight crime with an iron fist. However, his strategy could certainly backfire, leaving Guatemalans much worse off than they already are.