Adam Przeworski, a political scientist, left his native Poland a few months before the 1968 Prague Spring uprising and found he could not return home. To avoid being arrested as a dissident by the Communist government, he accepted a job abroad at a university in Santiago, Chile — only to watch his adopted country collapse into autocracy a few years later. In 1973, a violent coup installed a military dictatorship, led by Gen. Augusto Pinochet, wiping out Chilean democracy in one brutal stroke. “Nobody expected that it would be as bloody as it was,” Przeworski told me. “Or that it would last for 17 years.”

That shocking turn of events, in a country Przeworski thought he knew well, motivated him to find an answer to a seemingly simple question: Why do some democracies survive while others fall into autocracy? “That really was kind of an event that set my intellectual agenda for 50 years,” Przeworski, now an emeritus professor of politics at New York University, said recently.

For a long time, he and other experts believed that after a country had a few democratic handovers of power in a row and reached a certain level of wealth, then its democracy would be “consolidated” — safe from collapse. Once people could trust that free and fair elections would be held regularly, the theory went, other forms of politics would come to seem too costly and violent to consider.

The past decade has thrown that belief into question. The Jan. 6 attack on the United States Capitol marked the first time that America failed to peacefully transfer power from one president to the next. Last year, similar scenes played out in Brazil, as supporters of the outgoing president, Jair Bolsonaro, attacked federal buildings and called for a military coup. In Western Europe, far-right parties with policies and political styles similar to Donald Trump’s have gained in popularity, strengthening the sense that democracy could be vulnerable anywhere — even in places where it has long flourished.