MEXICO CITY — The seven-month-old baby Faith Marie Johnson survived hours without food and water in a bullet-ridden SUV in northern Mexico until her family members arrived. She had miraculously escaped the bullets flying around her. Her mother was among the three women and six children killed on Monday when gunmen ambushed their cars after they left a nearby Mormon community. Five other children survived being shot in the back, jaw, leg, wrist and chest. Those killed were all American citizens. The attack has sparked outrage from Mexico to Utah to the White House and focused American attention once again on violence south of the border.

Mexican prosecutors blamed cartels fighting a turf war over the lucrative routes smuggling drugs to Americans for the massacre. They said the Mormon families may have been targeted in a case of mistaken identity. Members of that Mormon community have also been outspoken against crime, including Julian LeBarón, who helped rescue the baby from the car. I found Mr. LeBarón to be one of the most inspiring speakers at a series of protests against violence I followed across Mexico in 2011. But the violence has only gotten worse since then.

President Trump responded to the attack on Monday with a series of tweets, calling the cartels “monsters” and offering to help defeat them. “The cartels have become so large and powerful that you sometimes need an army to defeat an army!” he wrote. “This is the time for Mexico, with the help of the United States, to wage WAR on the drug cartels and wipe them off the face of the earth.”

The president was correct that Mexico’s drug cartels are immensely powerful and engage in monstrous attacks on civilians, which have caused a humanitarian catastrophe here. But he did not say that the United States had already been helping to bankroll a Mexican military crackdown on traffickers since 2008 under a plan known as the Merida Initiative. And perhaps more important, he failed to mention that cartels sow their terror with American guns.