First we attacked the Russian soldiers with our gases, and then when we saw the poor fellows lying there, dying slowly, we tried to make breathing easier for them by using our own life-saving devices on them.

On 10 November during a visit to Copenhagen Hahn discussed these results with Niels Bohr and Lise Meitner. Hahn initially suspected it to be radium produced by splitting off two alpha-particles from the uranium nucleus. In 1917/18 Hahn and Lise Meitner isolated a long-lived activity which they named "proto-actinium".

He served as the last President of the Kaiser Wilhelm Society (KWG) in 1946 and as the founding President of the Max Planck Society (MPG) from 1948 to 1960. He is regarded as "the father of nuclear chemistry". Hahn was a courageous opposer of Jewish persecution by the Nazi Party and after World War II he became a passionate campaigner against the use of nuclear energy as a weapon.