Late on the night of March 8, 2008, a Mexican military patrol in the northern city of Chihuahua responded to neighbors' complaints about armed men. The soldiers, part of Mexico's ongoing effort to curb narco-trafficking violence, were met with a fusillade of grenades and gunfire. In the end, six men whom officials described as members of a drug gang lay dead.

On the government side, five soldiers were injured and one, Capt. David Mendoza Gómez, was killed. Mexican authorities...