It may be too early to say exactly what happened at Great Mills High School in Maryland, where a gunman this morning wounded two students. It’s not too soon, though, to wonder whether this will emerge as a vindication of the National Rifle Association’s Wayne LaPierre, who coined the formulation that “the only thing that stops a bad guy with a gun, is a good guy with a gun.”

The early report, which we read in USA Today, is that the attacking student who opened fire at the high school in southern Maryland injured two students shortly before classes were to begin. The newspaper quoted the sheriff of St. Mary’s County, Tim Cameron, as saying that the gunman was halted by the intervention of an armed school resources officer, who apparently went into action very quickly.

According the sheriff, the school resources officer exchanged shots with the gunman, who died at the scene. It was not clear whether it was the school resources officer who fired the shot that killed the attacker. Or what the attacker intended. Sheriff Cameron, though, told a broadcast that it was “unequivocally” the school resources officer who “stopped any further attack or assault on any other student.”

This strikes us as an important moment, coming, as it does, in the wake of Parkland, when parents, school officials, and legislators all over the country are intensifying their search for policy solutions. Nearly all of the proposals being offered by liberal think tanks and legislators are focused on narrowing the applicability of the Second Amendment guarantees of the right to keep and bear arms.

Mr. LaPierre’s prescription of a good man with a gun became famous in the wake of the horror at Sandy Hook, Connecticut, where a gunman slew twenty elementary pupils and six adults. The NRA chief horrified the Democrats. The then-editor of the New York Times’ editorials actually suggested that armed civilians would be more terrifying than a lone gunman firing into a classroom or movie theater.

“A police officer’s nightmare” is how the Times editor characterized idea of armed civilians intervening. It looks like the people of St. Mary’s County, Maryland, could come to a different view. We don’t yet know all the facts, including just how far the attacker was intending to go in the shooting at Great Mills. Early reports, though, suggest the facts will underscore the effectiveness of a good man with a gun.