If you wanted to tell the story of an entire Presidency in a single tweet, you could try the one that President Trump posted after Attorney General Jeff Sessions fired Andrew McCabe, the deputy director of the F.B.I., on Friday night.

Every sentence is a lie. Every sentence violates norms established by Presidents of both parties. Every sentence displays the pettiness and the vindictiveness of a man unsuited to the job he holds.

The President has crusaded for months against McCabe, who is a crucial corroborating witness to Trump’s attempts to stymie the F.B.I.’s investigation of his campaign’s ties to Russia. McCabe had first earned Trump’s enmity for supervising, for a time, the F.B.I.’s probe of Hillary Clinton’s e-mail practices, which ended without charges being filed against her. In these roles, McCabe behaved with the dignity and the ethics consistent with decades of distinguished service in law enforcement. He played by the rules. He honored his badge as a special agent. But his service threatened the President—both because of the past exoneration of Clinton and the incrimination of Trump, and for that, in our current environment, he had to be punished. Trump’s instrument in stifling McCabe was the President’s hapless Attorney General, who has been demeaning himself in various ways to try to save his own job. Sessions’s crime, in the President’s eyes, was recusing himself in the Russia investigation. (Doing the right thing, as Sessions did on that matter, is often a route to trouble with Trump.)

Sessions’s apparent ground for firing McCabe, on the eve of his retirement from the Bureau, thus perhaps depriving him of some or all of his retirement benefits, involves improper contacts with the news media. As an initial matter, this is rich, coming from an Administration that has leaked to the media with abandon. Still, the charges seem unfair on their face. After McCabe was dismissed, on Friday night, he said in a statement that the “investigation has focused on information I chose to share with a reporter through my public affairs officer and a legal counselor. As Deputy Director, I was one of only a few people who had the authority to do that. It was not a secret, it took place over several days, and others, including the Director, were aware of the interaction with the reporter. It was the type of exchange with the media that the Deputy Director oversees several times per week.” The idea that this alleged misdeed justifies such vindictive action against a distinguished public servant is laughable.

In his statement, McCabe spoke with bracing directness. “Here is the reality: I am being singled out and treated this way because of the role I played, the actions I took, and the events I witnessed in the aftermath of the firing of James Comey,” he said. In other words, McCabe was fired because he is a crucial witness in the investigation led by Robert Mueller, the special counsel. The firing of Comey is the central pillar of a possible obstruction-of-justice case against the President, either in a criminal prosecution or in an impeachment proceeding. By firing McCabe, Trump (through Sessions) has attempted to neuter an important witness; if and when McCabe testifies against Trump, he will now be dismissed by the President’s supporters as an ex-employee embittered by his firing. How this kind of attack on McCabe plays out in a courtroom, or just in the court of public opinion, remains to be seen.

What’s clear, though, is the depth of the President’s determination to prevent Mueller from taking his inquiries to their conclusion, as his personal attorney, John Dowd, made clear. In an interview with the Daily Beast, Dowd said, “I pray that Acting Attorney General Rosenstein will follow the brilliant and courageous example of the FBI Office of Professional Responsibility and Attorney General Jeff Sessions and bring an end to alleged Russia Collusion investigation manufactured by McCabe’s boss James Comey based upon a fraudulent and corrupt Dossier.” Of course, notwithstanding Dowd’s caveat that he was speaking only for himself, Rosenstein is on notice that his failure to fire Mueller might lead to his own departure. And Sessions, too, must know that his craven act in firing McCabe will guarantee him nothing. Trump believes that loyalty goes only one way; the Attorney General may still be fired at any moment.

To spin matters out further, Sessions could be replaced with someone already confirmed by the Senate—perhaps Scott Pruitt, the administrator of the E.P.A.—who could take office in an acting capacity. At the moment, Mueller’s investigation is supervised by Rosenstein, the deputy Attorney General, but presumably a new Attorney General, without Sessions’s conflict of interest, would take over that role. And that new Attorney General could fire Mueller. Such scenarios once seemed like the stuff of conspiracy theories. Now they look like the stuff of tomorrow’s news.

Andrew McCabe, who turns fifty on Sunday, will be fine as he moves to the next stop in his career. The demeaning and unfair act that ended his law-enforcement career will be seen, properly, as a badge of honor. Still, this is far from a great day for the men and women of the F.B.I., who now know that they serve at the sufferance of unethical men who think that telling the truth amounts to “sanctimony.” The lies in this story are about the F.B.I., not from the F.B.I. The firing of McCabe, and Trump’s reaction to it, has moved even such ordinarily restrained figures as John O. Brennan, the former director of Central Intelligence, to remarkable heights of outrage. Brennan tweeted on Saturday:

The haunting question, still very much unresolved, is whether Brennan’s confidence in America’s ultimate triumph is justified.