The American presidency is one of the strangest jobs in the world. Politicians aspire to it. The media focuses obsessively on the person who holds it and those who want to win it. Millions of books and articles have been written about it. And yet, most of the people who’ve held the job really weren’t that good. There are a few truly great presidents, some pretty good ones and a handful of complete disasters. But most can be summed up, as the Simpsons famously once did, as the “adequate, forgettable, occasionally regrettable, caretaker presidents of the U.S.A.” In fairness, the...