This ought to be Mr. Trump’s moment. After all, he ascended to the Oval Office not by abiding by conventional wisdom but by defying it. Right now we could use some of that defiance, because missing from the debate over mass shootings is a national leader willing to endure the opprobrium that comes with speaking a hard truth: There’s only so much the federal government can do here.

The American people, we are told, have grown cynical because we know the solutions but can’t implement them because of that pesky Second Amendment. But one driver of public cynicism surely has to be that little of what has been put in place so far has worked as promised. If America is serious about dealing with homicidal young men, the answer won’t be to dump the problem on local cops, the FBI or some federal database—especially when no crime has been committed.

“Everything’s on the table,” Mr. DeWine told reporters after the Dayton killings. But that really isn’t true. Like so many other pols, the governor means he’s willing to consider legislation controlling guns or access to them. What’s almost certainly not on the table? A critical look at whether more extensive background checks or red-flag laws or bans on “assault weapons” will in fact solve this problem.