Curators at the Museum of the History of Polish Jewsnoticed a change among their visitors earlier this year: More people were pushing back against its version of history. Some asked why there was no mention of Jews selling out their neighbors to various enemies over the centuries. Others questioned whether Poles were really involved in a notorious World War II massacre. Anti-Semitism, it seemed, was acceptable again. Museum guides had to be trained to handle the verbal aggression.

“The dynamic changed overnight,” says Dariusz Stola, the history professor who runs the museum, observing that prejudices apparently had free rein in the wake of a proposed law. “The problem is that young people get used to hate speech. Some people don’t like chips, some people hate Coca-Cola—and some people hate the Jews.”

It’s a jarring piece of recidivism, set amid Poland’s economic boom. Every few months, a new glass building or office complex expands Warsaw’s skyline, which is anchored by the Palace of Culture and Science, the sand-colored gift from Stalin in the 1950s. Young office workers in smart skinny trousers and New Balance running shoes zip around on the city’s new cycle lanes. Poland’s $470 billion economy is expanding at more than 5 percent a year, based on its performance in the latest quarter, about double the neighboring euro area. A journey on an Italian-built high-speed train south from Warsaw to the old mining city of Katowice is like traveling through the pristine French countryside. New highways and railways funded by the European Union crisscross the country. Local manufacturers are thriving as they help keep the German industrial machine across the border stocked with parts and components.

This seemingly inexorable transformation into a showcase of European integration has been thrown into reverse by Poland’s culture wars. The trigger was a proposed law making it a crime to suggest Poland was in any way responsible for the Holocaust. A two-year-old populist government decided that Polish honor needed to be protected, so it set to rewriting history, opening a Pandora’s box of anger and nativism.