What happened at Sandy Hook was not the failure to plan; it was the failure of the plan. The teachers and administrative staff executed their school district’s plan heroically in trying to save lives, some at the loss of their own. Police departments changed their policies after Columbine and now rush to the source of an incident inside a school building at great risk to themselves. But a major flaw in such plans persists to this day—namely that it takes just a few unguarded minutes for a catastrophe to unfold.

I have no desire to turn my children’s school into an armed camp. But I firmly believe that had there been armed, trained security personnel anywhere near the front entrance of Sandy Hook Elementary on the morning of Dec. 14, Adam Lanza either never would have approached the school or his attack would have had a radically different outcome.

One-third of the nation’s elementary, middle and high schools reportedly already have armed security on campus. In 2000, President Clinton marked the one-year anniversary of Columbine by proposing a significant expansion of the government’s existing “COPS in Schools” program. Now that the National Rifle Association’s Mr. LaPierre has made a similar proposal, he is being ridiculed. Why?