We’re still trying to get a clear sense of the legal mechanics of Sessions de facto firing. There are legal particulars about the President’s powers depending on whether a cabinet secretary resigns or is fired. What seems clear is that this is the beginning move in ending the Mueller probe or, at least for now, putting it under the control of a Trump loyalist who is on the record saying that he believes the investigation is in fact illegitimate. The White House knows Mueller’s office is or was about to come out of pre-election hibernation and likely move to a series of new indictments.
We need to remember the context. The President and members of his entourage conspired with a foreign power to ascend to the presidency and has spent the bulk of his two years in office trying to evade the law. Rod Rosenstein had been suggesting to some of his more apolitical colleagues at the DOJ that they should be prepared for dramatic changes in leadership in the first days of November.