How can a historian escape the claim that the world became worse once the United States became the leading power?
Short answerCompare a much longer historical record: human behavior is broadly persistent even though the tools and systems available to people change.
NotesA longer span of history is needed before accepting a simple claim that humanity became better or worse under one leading power. Human motives and behavior remain broadly recognizable even as institutions and technologies change. People remain tool-using animals whose means alter more readily than their underlying impulses. This distinction separates evidence-based comparison from a politically attractive periodization.