Noted Holocaust historian and Nazi-Germany refugee Fritz Stern has died at the age of 90. Stern, a longtime Columbia University professor, died Wednesday at his home in Manhattan.

Stern was born in what is now Poland, to a Jewish-convert family. The family fled Europe in 1938 after the rise of Hitler, a life lesson that Stern applied to his trove of writings on the subject. He taught European history for four decades, earning a PhD from the university that would also become his place of work.

After retiring from Columbia in 1997, he continued writing his essays and books, occasionally advising government officials like Margaret Thatcher.

“Fritz Stern rightly demanded of us Germans that the crimes against the Jews be preserved in our collective memory, to honor the victims, to learn from this rupture of civilization, and to develop standards for shaping the present,” wrote German President Joachim Guack, in a condolence message to Stern’s family.