Robert Mueller looks to be approaching his endgame, and the press is alive with speculation as to how bad the special counsel’s report will be for President Trump. It’s also worth noticing how good Mr. Mueller has been to another president, Barack Obama, and his team.

This benevolence was on display in the sentencing document the special counsel’s office filed on Tuesday for Mike Flynn. The former Trump national security adviser, we are told, has provided “substantial assistance,” sitting for 19 interviews that aided in “several ongoing investigations.” In light of this help, the Mueller team recommends leniency—perhaps even sparing him prison time.

That would be just. The special counsel provides no evidence Mr. Flynn was part of any collusion with Russia. He was instead brought up on a charge of lying to the Federal Bureau of Investigation. And it’s not even clear he did that. The FBI officials who first interviewed Mr. Flynn didn’t think he lied about his interactions with Russian Ambassador Sergei Kislyak. More likely, Mr. Flynn copped a plea to save his son from worse treatment at Mr. Mueller’s hands.

But what about the potential crimes that put Mr. Flynn in Mr. Mueller’s crosshairs to begin with? On Jan. 2, 2017, the Obama White House learned about Mr. Flynn’s conversations with Mr. Kislyak. The U.S. monitors phone calls of foreign officials, but under law they are supposed to “minimize” the names of any Americans caught up in such eavesdropping. In the Flynn case, someone in the prior administration either failed to minimize or purposely “unmasked” Mr. Flynn. The latter could itself be a felony.

Keep reading Kimberley Strassel's column in the Wall Street Journal.