Only Donald Trump’s inauguration in 2017 could have been a sweeter day for the present occupants of the White House than last Sunday. The publication of Attorney General William Barr’s letter summarizing special counsel Robert Mueller’s report lifted an enormous political weight off President Trump and his team.

Mr. Mueller found no evidence that “the Trump campaign conspired or coordinated with the Russian government in its election interference activities.” Mr. Barr concluded “the evidence developed” during the Mueller investigation...