She then assisted historian Jacques Freymond with his research for the book "Le conflit sarrois, 1945-1955".
[3] She participated in the founding of the journal Relations internationales in 1972, and took the co-presidency of the editorial committee with Pierre Guillen in 1982.
Steiner researched the history of Nazi Germany, publishing among other things a biography of Adolf Hitler.
[4][5] She also promoted a non-orthodox thesis contending that Hitler, shortly prior to his suicide in Berlin in April 1945, had a change of heart and concluded that the policies of the Third Reich were disastrous, and for that reason appointed Admiral Karl Dönitz as his successor rather than any member of the Nazi bureaucracy which he cultivated, with the hope he would lead a more accommodating policy towards the Allied Powers.
She also claimed that Admiral Dönitz, during the short-lived Flensburg Government in May 1945 hoped to lead postwar Germany with the purpose of conducting peaceful policies that will not repeat the policy of invading other countries and holding foreign populations as slave labor.