Thanks to the work of special counsel Robert Mueller, we know that the Russian operatives who tried to interfere in the 2016 election “did not have the cooperation of President Trump or the Trump campaign—or the knowing assistance of any other Americans,” Attorney General William Barr said Thursday. But Mr. Mueller should never have accepted the assignment to begin with.

Justice Department regulations authorize appointment of a special counsel when the attorney general “determines that criminal investigation of a person or...