Extravagance can be intoxicating, and those who grow accustomed to extravagance, only to be deprived of it, can miss it terribly. That accounts for much of the powerful hold John F. Kennedy has on a generation of Americans even today. He led people to imagine that their government had the boundless capacity to improve the world, and on the day he died, they could still believe that. His administration and that of his vice president and successor Lyndon B. Johnson are significant in the same way: They represent the pinnacle of ambitious, visionary government. What each president lacked was a...