The mid-1930s to the late 1940s saw the emergence of a coherent nuclear reaction framework that linked neutron-induced radioactivity, the fission mechanism, and resonance phenomena into a unified theory of nuclear dynamics. The field advanced through the use of the liquid-drop model to describe shape distortions in fission, the recognition of slow-neutron resonance behavior in capture processes, and the application of dispersion-like relations and symmetry-based considerations to explain cross sections and spectroscopic patterns. This period solidified the paradigm that nuclear reactions could be understood through a combination of structural and reaction-theory concepts, guiding early reactor development and nuclear chemistry.