"Populist rage, ideological fracture, economic and technological shocks, war, and an international system studded with catastrophic risk—the early decades of the twenty-first century may be the most revolutionary period in modern history. But it is not the first. Humans have lived, and thrived, through more than one great realignment. What are these revolutions, and how can they help us to understand our fraught world? Fareed Zakaria investigates...