“[In April 2010] I was stunned by a terrible tragedy: the crash of a plane that had carried the Polish president, Lech Kaczynski. He had been flying to the Russian city of Smolensk to visit the memorial at Katyn, where Stalin murdered 20,000 Polish officers in 1940. Several dozen senior military figures and politicians were also on the plane, many of them friends of mine and colleagues of my husband, who was then the Polish foreign minister. Among them was his deputy, Andrzej Kremer, a wonderful man and brilliant diplomat. In the sweep of emotion that followed the crash, comparing the event to Katyn, I wrote [in my op-ed at the time]: “This time around, nobody suspects a conspiracy.” As an excuse, I offer the fact that the tragedy initially seemed to bring people together. Politicians of all parties, from right to left, had been on the plane. Widely attended funerals were held across the country. Even Vladimir Putin, then the Russian prime minister, seemed moved. He arranged for the broadcast of “Katyn”—an emotional and very anti-Soviet Polish film—on Russian state television as a kind of memorial. Nothing like it has ever been shown so widely in Russia, before or since.”

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“But my optimism was premature. The president’s brother, Jaroslaw Kaczynski, then the unpopular leader of the parliamentary opposition, seems to have initially believed, as all the evidence has always shown, that the crash was an accident. Then he changed his mind. Perhaps he could not accept that his beloved twin had died randomly, in a pointless crash. Perhaps he was maddened by grief. Perhaps he felt guilty: He had helped plan the trip. Or perhaps, like Donald Trump, he saw that a conspiracy theory could help bring him to power.”

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“Much as Trump used birtherism to inspire his core voters, Kaczynski, in the years that followed, used the Smolensk crash to motivate his supporters, that minority of the Polish population that remains convinced that unnamed secret forces control the country, that the “elite” is manipulated by foreigners and that everything that has happened in the country since 1989 is part of a sinister plot. And it worked. Last year, thanks to flukes of the electoral system, less than 40 percent of the vote—reflecting 18 percent of the adult population—proved sufficient for his nationalist-populist party, Law and Justice, to win a slim parliamentary majority.”

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“I realise that there is far more detail here about Poland than most non-Polish readers care to know. But I’m offering it for a reason. Trump, like Kaczynski, pushed a patently false conspiracy theory hard for many years, despite the utter lack of evidence. Last week, he found it expedient to discard that theory, but once he is president, he might find it expedient to adopt it again—or perhaps to push one of the many others he has championed. As president, he can then use the state—the Justice Department, the security bureaucracy, the FBI—to pursue them. A Trump administration could make birtherism the excuse for fake investigations, hearings and even trials that would do terrible and irreversible damage to US politics and the rule of law.

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It all sounds unthinkable, of course. But if you’d asked me five years ago, or even one year ago, I would have told you that the transformation of the Smolensk conspiracy theory into state ideology was unthinkable, too. And yet it has come to pass.”

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Read: In Poland, a Preview of What Trump Could Do to America