Develops and discusses a theory of alternating long waves of globalization and deglobalization based on changing marginal costs and benefits of internationalization. Provides historical data for this long wave. Argues that an optimal level of globalization exists (which is not the maximally possible) and shows how limits to openness are endogenously enforced, both domestically (costs of redistribution) and internationally (costs of hegemonism). Argues that Make America Great Again and Brexit are symptoms but not causes of deglobalization. Investigates motives and reasons for disagreement on the current phase of actual and measurable deglobalization based on the Black Swan character of the Great Recession. Discusses policy implications.