While the decision will provide some short-term political relief, not least for Mr. Rosenstein, it also opens up years of political risk to the Trump Administration with no guarantee that the public will end up with any better understanding of what really happened.

The problem with special counsels, as we’ve learned time and again, is that they are by definition all but politically unaccountable. While technically Mr. Rosenstein could fire Mr. Mueller if he goes too far, the manner of his appointment and the subject he’s investigating make him de facto untouchable even if he becomes an abusive Javert like Patrick Fitzgerald during the George W. Bush Administration.

What the country really needs is a full accounting of how the Russians tried to influence the election and whether any Americans assisted them. That is fundamentally a counterintelligence investigation, but Mr. Mueller will be under pressure to bring criminal indictments of some kind to justify his existence. He’ll also no doubt bring on young attorneys who will savor the opportunity to make their reputation on such a high-profile investigation.