Former Vice President Joe Biden, now a 2020 Democratic presidential contender, has locked into a specific story about the controversy in Ukraine. He insists that, in spring 2016, he strong-armed Ukraine to fire its chief prosecutor solely because Biden believed that official was corrupt and inept, not because the Ukrainian was investigating a natural gas company, Burisma Holdings, that hired Biden's son, Hunter, into a lucrative job. There’s just one problem. Hundreds of pages of never-released memos and documents — many from inside the American team helping Burisma to stave off its legal troubles — conflict with Biden’s narrative. And...