For all their disagreements, the left and the right concur on one thing: The world is getting worse. Whether the decline is visible in inequality, racism and pollution, or in terrorism, crime and moral decay, both sides see profound failings in modernity and a deepening crisis in the West. They look back to various golden ages when America was great, blue-collar workers thrived in unionized jobs, and people found meaning in religion, family, community and nature.

Such...