Rothfels was noted for his claim that Bismarck was neither the "iron chancellor" of "banal legend" nor an "opportunist", but rather a profoundly religious man struggling to deal with a reality whose full complexity was only understandable to God.
[citation needed] A reactionary in his politics, Rothfels was hostile towards the Weimar Republic,[4] through combination of authoritarianism and mass national movement, he hoped, it would be destroyed, and connections with Western democracies broken,[4] and envisioned that on ruins of this state a new Reich would emerged formed out of East Prussian Baltic Northeast and Southeastern outposts of former Habsburg Empire.
[5] As a historian, his major interests were Otto von Bismarck, Clausewitz, and later on, the conservative German opposition to Adolf Hitler.
A major interest of Rothfels in the 1920s was his belief in the obsolescence of the nation-state, and the need for a "loosening up" of the Versailles borders through increased protection of minorities.
[6] Rothfels promoted an idea of race classification based on readiness of non-German ethnic groups in Eastern Europe to submit themselves to rule of German Third Reich.
[4] Non-Germans would have been subject to hierarchical employment conditions and essentially have status of indentured workers, based on racist criteria.
[7] Eventually, Rothfels was forced to leave his university position due to his Jewish ancestry, despite intervention by Hermann Rauschning, the Nazi president of Danzig Senate, and Theodor Oberlander, director of League of German East (Bund der Deutschen Osten) and NSDAP's East Prussian intelligence agency,[4] and was forbidden to teach a year later.
What decided the issue for him was his experience during the Kristallnacht pogrom when his house was looted and trashed by the SA and he himself was arrested and held by the Gestapo for several hours, during which he was deprived of his crutches and beaten up.
Together with his wife and their three children, Rothfels left for the United Kingdom, where he hastily began to learn English, a language that he subsequently mastered.
Most notably, Rothfels portrayed Clausewitz as a man under considerable psychological strain caused by his commoner background in the largely aristocratic Prussian Army.
In 1948, Rothfels published his most famous book, The German Opposition To Hitler, which celebrated those conservatives who attempted the 20 July plot of 1944, which was based upon a lecture given at the University of Chicago in 1947.
Rothfels argued that the actions of the 20 July conspirators were motivated a sense of noblesse oblige, devotion to the principles of Christianity and the highest form of patriotism.
Rothfels wrote "In many respects, Nationals can be considered as the final summit of an extreme consequence of the secularization movement of the nineteenth century".
[10] Rothfels argued that the Nazis came to power as a result of a series of unfortunate developments that had occurred in Germany after World War I such as the Great Inflation of 1923 and the Great Depression, and often criticized those in his view promoted the view that sought to equate Deutschum with Nazism such as Sir Lewis Bernstein Namier, William L. Shirer, A.J.P.
In particular, he broke new ground by publishing Kurt Gerstein's reports relating to the Final Solution in the first edition of the Vierteljahrshefte für Zeitgeschichte in 1953 and another article in 1959 that examined the plight of Polish Jewry under Nazi rule.
In 1954, he and one of his star pupils from the University of Chicago, Gerhard Weinberg had a renowned debate on the pages of Vierteljahrshefte für Zeitgeschichte with Andreas Hillgruber and Hans-Günther Seraphim over the issue of whether the German invasion of the Soviet Union in 1941 had been a "preventive war" forced on Adolf Hitler by the possibility of Soviet attack on Germany.
Later, in 1961, Rothfels took a strong stand against the American neo-Nazi historian David Hoggan who claimed that the outbreak of war in 1939 had been due to an Anglo-Polish conspiracy against Germany.
Another area of interest for Rothfels was the expulsion of the ethnic German population from Eastern Europe after World War II.
In the 1950s, Rothfels worked with Theodor Schieder, Werner Conze and other historians to produce the multi-volume Documentation of the Expulsion of Germans from East Central Europe.
The historian Ingo Haar in his 2000 book Historiker im Nationalsozialismus called Rothfels an enemy of the Weimar Republic and a Nazi sympathizer.
They soon found themselves together in a commission led by Schieder that was set up by the government for documenting the expulsion of Germans after World War II.