The worries and warnings come from across the political spectrum and across the oceans. New York Times critic Nicolai Ouroussoff calls America “an empire enthralled with its own power and unaware that it is fading.” Former Clinton administration official Charles Kupchan concludes that “American primacy is already past its peak.” According to Joseph Nye, who served under Presidents Carter and Clinton, America’s “soft power — its ability to attract others by the legitimacy of U.S. policies and the values that underlie them — is in decline.” Peggy Noonan, speechwriter for the most optimistic of presidents, Ronald Reagan, asserts that “in...