LONDON — On December 31, 1999, as the new millennium dawned, writer and historian Anne Applebaum threw a party at her small manor house in Chobielin, northwest Poland.

She and her husband, Radek Sikorski, then a deputy foreign minister in Poland’s center-right government, felt they and their guests had much to celebrate. Communism had been vanquished, democracy and free markets were in the ascendant, and Poland was on the verge of joining the West.

But, as Applebaum says in her new book “Twilight of Democracy: The Failure of Politics and the Parting of Friends,” that mood has now soured.

“Nearly two decades later, I would now cross the street to avoid some of the people who were at my New Year’s Eve party,” she writes. “They, in turn, would not only refuse to enter my house, they would be embarrassed to admit they had ever been there. In fact, about half of the people who were at that party would no longer speak to the other half.”

Her guests — mostly Poles, but supplemented by a smattering of friends and colleagues from London, Moscow and New York — have been split by what Applebaum terms a “profound divide.”

It is one which, albeit with local variations, has splintered European politics, as well as those of Applebaum’s native United States. Some, such as Applebaum and her husband, have remained loyal to their center-right roots. But others have taken a very different turn and embraced the hard-right populist and authoritarian parties which now dominate Hungary and Poland, compete for power in France, Italy and Spain, and have reshaped the political landscape in Britain and the US.

While Applebaum writes that the estrangements are “political not personal,” in truth they are both. One of her millennium eve guests now spends her time spreading anti-Semitic conspiracy theories — including that Jews were responsible for the Holocaust — on social media. Another is a frequent guest on Polish state television who attacks Jews as “scabby” and “greedy,” calls Jewish organizations “blackmailers,” and has recanted his previous support for Israel. And a Hungarian former colleague is an avid propagandizer of the attacks on the Jewish billionaire philanthropist George Soros which are peddled by Viktor Orbán’s government.