The slaughter Monday of three Mormon women and six children, all American citizens who were longtime residents of Mexico, brings home a cruel reality of America’s neighbor to the south. Drug gangs control huge swathes of the country, and the government in Mexico City is too often overwhelmed by the criminal firepower and money.

The women and children were attacked by gunmen as they traveled in SUVs in the northern state of Sonora in broad daylight. Mexican officials said Tuesday that it could have been a case of mistaken...