A refugee from Nazi Germany after opposing Hitler's rise to power, von Klemperer fled Germany in 1938. He worked as an intelligence officer in the U.S. Army during the war and settled in America afterward.
The New York Times said von Klemperer's seminal work as a historian came in a book published in 1992, "German Resistance Against Hitler: The Search for Allies Abroad, 1938-1945." Colleagues said the work broke open for the first time the domestic effort by some prominent theologians, diplomats and military leaders to undermine Hitler's rise to power and efforts to gain assistance from foreign governments to do so.
The effort culminated in an attempt to kill Hitler with a briefcase bomb in the map room at Wolfsschanze, Hitler's East Prussia bunker headquarters. The Nazi dictator survived the blast claiming divine intervention and set about purging his inner circle of advisers he believed were conspirators.
After the war, von Klemperer earned his doctorate in history from Harvard. He was a professor emeritus at Smith College, the Times said.