The best witness for the never-completed prosecution of President Trump, it turned out, was the prosecutor himself: Jack Smith. Last week, the House Judiciary Committee released the transcript and video of Smith’s deposition, which is a stark reminder of the cost of letting Trump go unpunished and the danger of the President’s revenge-seeking against those who sought to hold him to account.

The best witness for the never-completed prosecution of President Trump, it turned out, was the prosecutor himself: Jack Smith. Last...