It would be naive to pretend that former presidents have always been paragons of compassion and politesse. Richard Nixon’s private conversations bristled with antisemitism and misogyny. Lyndon Johnson used the n-word with abandon, according to his biographer Robert Caro, although he tenaciously pushed through landmark civil rights legislation. Even Trump’s more emollient predecessor, Joe Biden, had moments of bone-chilling cynicism. Richard Holbrooke, a former special envoy to Afghanistan and Pakistan, recalled warning Biden, at the time vice-president, in 2010 that if the US pulled out of Kabul it would be abandoning Afghan women and girls. “F*** that, we don’t have...