When Dwight Eisenhower coined the phrase “domino effect” in 1954, the French had just been defeated in Vietnam at Dien Bien Phu—and the president sensed that the fall of Indochina to communist forces would trigger a series of similar events. “You have a row of dominoes set up,” said Ike. “You knock over the first one. . . . What will happen to the last one is the certainty that it will go over very quickly.”

In the immediate aftermath of the British vote to leave the European Union, many analysts warned of a similar domino...