The denouement of the Special Counsel investigation — unfolding on the Imax of the Internet — illuminates nothing so much as the fact that our republic is in a crisis of separated powers. It shows that a special counsel can come close to destroying a presidency even in cases where, as now seems clear in respect of President Trump, there was neither collusion with a foreign power nor obstruction of justice.

This is what happens when our press, politicians, and prosecutors ignore the constitutional principle of separated powers and press for or set up either an independent or special prosecutor. Such a blunder destroyed Richard Nixon’s presidency, damaged President Reagan’s, nearly kneecapped President George H.W. Bush’s, and almost destroyed Bill Clinton’s presidency.

Yet rarely has the crisis of separated powers been thrown into such sharp relief as in the case of Mr. Trump. Feature the press conference at which Attorney General Barr presented Robert Mueller’s report. The moment came as Mr. Barr was being questioned in respect of Mr. Mueller’s decision neither to prosecute nor to exonerate the President in respect of obstruction of justice. Mr. Mueller left it to Mr. Barr.

The special prosecutor did so, Mr. Barr reported, not because of Justice Department policy against indicting the president. Rather Mr. Mueller couldn’t determine that there was a crime, though he came close. Mr. Barr then decided not to pursue charges. When a reporter asked Mr. Barr why, the AG explained the nature of the prosecutorial function — only to be asked this astonishing question:

“Did the special counsel indicate that he wanted you to make the decision or that it should be left to Congress?” That is, after two generations of debate about special counsels and separated powers, a reporter sitting in the front row of a press conference in the Justice Department asked whether the special counsel wanted Congress to make the decision on whether to prosecute.

Mr. Barr said that Mr. Mueller had not indicated that his purpose was to leave the decision to Congress. How could he have? The Constitution grants to Congress a whole list of powers — to, say, levy taxes, declare war, regulate commerce, coin money. Nowhere, though, does it grant Congress the power to prosecute crimes or decide whether they should be prosecuted.

On the contrary, it specifically grants the prosecutorial power — and duty — to the president alone. It does so in Article II, where it says of the President: “He shall take Care that the Laws be faithfully executed, and shall Commission all the Officers of the United States.” The idea of the Congress making a decision on whether to prosecute a President, or anyone else, would have floored the Founding Fathers.

It’s not just the press that is floundering on this head. No less a figure than Neal Katyal is plumping for Congress to step in. Mr. Katyal, who as an aide to President Clinton wrote the special counsel regulations, suggested today on Savannah Guthrie’s broadcast that Mr. Mueller thought Congress ought to have handled the decision on whether to prosecute. Mr. Mueller can, and no doubt will, speak for himself.

Just to be clear, though, Congress has no power to prosecute a crime. It may be that what Mr. Katyal wants is for Congress to impeach Mr. Trump. The House certainly has that power. Even then, though, the only thing impeachment can accomplish, if the Senate convicts, is removing, and further disqualifying, a president from office. Any subsequent criminal prosecution could be decided by, and only by, the executive branch.

Most amazing is the fact that the only one in this whole drama who seems to grasp the seriousness of separated powers is President Trump. It is why Mr. Trump was so furious, even crazed, at the appointment of a special counsel. Mr. Trump knew how wrong it was that Mr. Mueller was beyond his control politically, if not technically, even while the special counsel exercised powers belonging to the president.

No wonder Mr. Trump has been vowing, as he did again today, that this sort of thing should never again happen to a president. A top-to-bottom reform of the regulations so as to address that point would be the best thing to come out of this tragedy. President Trump and Attorney General Barr have the chance now to protect our next and future presidents, so that they shall never more be separated from the powers properly granted only to them.