His father was journalist Fritz Klein, Sr., editor of the
Deutsche Allgemeine Zeitung from 1924 to 1933, when he was fired by the Nazi government. The younger Klein served as a soldier in
World War II. After 1945, he opted for
communism, joining the
Communist Party in the
GDR. In 1947 he married Dorle Deiters, with whom he had a son, Wolfgang. He enrolled at
Humboldt University in Berlin in 1946, and received his doctorate in 1952 with the thesis
Die diplomatischen Beziehungen Deutschlands zur Sowjetunion 1917 - 1932. (The Diplomatic Relationship of Germany to the
Soviet Union 1917 - 1932.). After his graduation, he was appointed as editorial secretary and later as editor of the "Zeitschrift für Geschichtswissenschaft" ("Journal of Historical Studies"), but was removed in 1957 for political reasons, but was asked to take the post again in 1990. He worked for more than 30 years as a professor and researcher at the Institut für Geschichte of the Deutsche Akademie der Wissenschaften (Institute of General History at the
German Academy of Sciences at Berlin). During his tenure there, he supervised the publication of the three volume
Deutschland im Ersten Weltkrieg, which
Roger Chickering called "the richest and most comprehensive account of Germany in the First World War". Klein was an active part in the Fischer Controversy, around
Fritz Fischer's
Germany's Aims in the First World War, which argued that Germany was responsible for instigating
World War I. ==Selected publications==