Actually, the letter was not a cipher. Plan D was the fourth of several studies organized by an opaque advocacy group, known as the Hub, to prepare for the depredations of the Trump era. The Hub is known in Washington for its sophisticated dark-money interventions in electoral politics. During the 2020 campaign, it also gathered up strategists, lawyers and activists to draft plans for a different kind of conflict. The document is an artifact from a dangerous time: Warning that Trump would surely not concede defeat to Joe Biden, it advised Trump’s opponents to “assume the worst” would follow. It...