The presidency of Donald J. Trump is a grand enigma, so enigmatic that powerful opponents — and even some supporters — thought he must have had outside help to win the Oval Office. The Mueller investigators spent more than $30 million over two years searching for Russian collusion and produced almost 450 pages trying to solve the Trump enigma but found no collusion. Impeachment proceedings then feverishly sought reasons to remove Trump from office. The maddening (to some) mystery remained: What power made Donald Trump the president of the United States of America? “How?” is indeed at the heart of...