[2] 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.
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.
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.
[3] 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".
[4] 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.