Patrick Allitt says that the infuriating but reassuring can-do spirit that once defined the United States is finally dying out. But what will we all do when it’s gone? The first time I went to America, in 1977, I couldn’t believe how cheerful, peppy and purposeful everyone was. The late seventies were bad years by American standards, the Jimmy Carter era of stagflation and malaise, but to someone coming out of Jim Callaghan’s Britain the place seemed almost insanely upbeat. Strangers would greet you enthusiastically, with a ‘How ya doin?’ in New York and a ‘Have you taken Jesus as...