There is a long tradition of reading a nation’s character in the way it marks its own milestones. When the United States turned 150, Philadelphia raised an 80-foot-tall luminous replica of the Liberty Bell and hosted a six-month Sesquicentennial International Exposition. When it turned 200, a country shaken by Vietnam, Watergate and political assassinations nonetheless found in its bicentennial a moment of genuine collective catharsis: a million people on the National Mall, the Freedom Train...