When Poland was overrun, Rotblat made the decision to work on the bomb. His belief was that the allied scientists needed to do this in order to make it possible for the bomb not to be used. In other words the reasoning goes: if it is possible for Hitler to have the bomb, then the only way we can prevent him from using it (against us) would be by us having it too (without using it).

Martin Underwood, Joseph Rotblat: a man of conscience in the nuclear age, Brighton [England] ; Portland, Or, 2009, p. 24