WHEN he took office as Mexico's president in December 2006, Felipe Calderón was faced with the unchecked and violent power of drug gangs and a corrupt and ineffective set of police forces. So he turned to the army, sending some 45,000 troops to patrol the streets in the most troubled cities. It was supposed to be an emergency measure. But the troops are still there, and since they are not trained for policing duties, perhaps inevitably from time to time there are allegations that they abuse their powers. What is less inevitable is that their crimes seem to be going unpunished.

Human Rights Watch, a New York-based group, has investigated 17 such incidents involving 70 victims. Its allegations, contained in a report released this week, are backed by eyewitness accounts and forensic evidence. They include gruesome tales of killings, torture and rape of people with no ties to the drug trade. Many of these cases had already been documented by Mexico's own government-backed human-rights commission, which has seen a fourfold rise in complaints against the army since Mr Calderón took office. But that body has no power to act on its findings.

Nearly all criminal cases against soldiers are handled by military courts, where they are treated as disciplinary matters rather than crimes. Military judges are appointed by the defence secretary, who can remove them at will. Their verdicts are not made public and victims cannot appeal to civilian courts. When asked by Human Rights Watch to provide details of any cases in which soldiers had been convicted of crimes, the defence ministry did not do so.

Mr Calderón has made reforming the judicial system a priority, winning approval for a constitutional amendment that makes the civilian courts more open and less cumbersome. But he has not touched their military counterparts. Polls show the army is popular and trusted. “Having military officers commit abuses is seen as the lesser evil,” says Raúl Benítez, a defence specialist at the National Autonomous University of Mexico. “People just want the army to protect them from the narcos.” Since he is relying so heavily on the army, it is hard for Mr Calderón to force it to submit to the law. But unless he does, he may face an overmighty force, and one that starts to lose the trust of Mexicans.