David Frum lays out all the details in "Stone Walks Free in One of the Greatest Scandals in American History" in The Atlantic.
Stone was accused of—and convicted of—lying to Congress about his role in the WikiLeaks matter. Since Stone himself would have been in no legal jeopardy had he told the truth, the strong inference is that he lied to protect somebody else. Just today, this very day, Stone told the journalist Howard Fineman why he lied and whom he was protecting. “He knows I was under enormous pressure to turn on him. It would have eased my situation considerably. But I didn’t.” You read that, and you blink. As the prominent Trump critic George Conway tweeted: “I mean, even Tony Soprano would have used only a pay phone or burner phone to say something like this.” Stone said it on the record to one of the best-known reporters in Washington. In so many words, he seemed to imply: I could have hurt the president if I’d rolled over on him. I kept my mouth shut. He owes me.Read all the rest of the filthy details.
Trump should be impeached over this.