They came like rapid-fire rifle shots. On Friday, former national security adviser Mike Flynn copped a deal, pleading guilty to making false statements to the FBI and promising to cooperate with Special Counsel Robert Mueller in return for avoiding more-serious charges.

Then on Saturday, President Trump sent his most ill-advised tweet yet, saying he knew all along that Mr. Flynn had lied to the FBI. This was a change from the president’s previous explanation for ousting Mr. Flynn—namely, that he had misled Vice President...