Donald J. Trump’s profoundly stupid boast today underscores perfectly a point I sought to make in an earlier blog post.

The president assumed the role of “armchair hero” in declaring — during a White House gathering with the nation’s governors — what he would have done had he been present at Marjory Stoneman Douglas High School in Parkland, Fla., on Valentine’s Day.

He said he would have stormed into the building when the gunman opened fire, killing 17 students and educators.

Really, Mr. President? This is the kind of idiotic nonsense that drives many of us nuts.

He was speaking about former Broward County Sheriff’s Deputy Scot Peterson, who reportedly stood by — frozen — while the gunfire erupted inside the high school. He called the deputy’s conduct unacceptable. On that score, the president is correct.

However, to insert himself into a tragedy and assert how he would have acted — days after the fact — speaks to a curious form of projection from someone who one would think should know better.

There can be plenty to say about the deputy’s lack of action when it was need in the moment. We haven’t yet heard from him. I am one who hopes he is able and willing to explain why he didn’t do what he was trained to do.  I don’t expect him to offer a plausible explanation, but the nation has a need to hear his version of events.

Americans do not need to hear senseless boasting from a president who was nowhere near the tragedy when it struck — and broke everyone’s heart.