(Much of this section is based on an article "Categorization, Anomalies and the Discovery of Nuclear Fission" by H. Andersen in "Studies in History and Philosophy of Modern Physics, Vol 27, 463-492, 1996 See also Chapters 9 to 11 of R. Rhodes, "The Making of the Atomic Bomb", Touchstone, New York, 1986.)
It is well known that the discovery of nuclear fission in 1938 had far reaching, even revolutionary effects on world politics. It certainly led to a change of thinking about warfare. Less known is the fact that the discovery of nuclear fission involved a major change of thinking about nuclear reactions, though not of the scale that relativity or quantum mechanics led to. Let us go back to the 1920's. At that time, many atomic systems became understood with the help of quantum mechanics. However, little was known about nuclei, except some rough information about nuclear binding,and the existence of some natural radioactivity.
Before the discovery of the neutron in 1932, there was no nuclear model. It was believed that nucleons are made of protons and electrons only. However,there was one success of quantum mechanics in nuclear physics: the theory of alpha decay. As was shown by Gamow, the long lives of alpha-decaying substances could be understood in terms of the alpha particle having to tunnel through a potential barrier, due to the Coulomb interaction between the alpha particle and the rest of the nucleus. Of course, in classical physics, tunnelling cannot occur as it involve a temporary violation of energy conservation. Even in quantum mechanics, such tunnelling only occurs with appreciable probability for light particles.
Thus it became part of the conventional wisdom that alpha particles are the heaviest particles that can be emitted in nuclear reactions. While this was reasonable enough, the community of nuclear physicists was reluctant to give it up, even as evidence (from fission) suggested something else. In particular, the discovery of the neutron in 1932 ushered in the age of modern nuclear physics, but it did not lead to a change of thinking about the alpha being the heaviest particle that can be emitted.
In 1934 Fermi studied the reactions induced by slow neutrons on many different nuclei. Among other things, this led to the production of a large number of new isotopes, some subject to beta-decay. An example is the reaction:
n + 27Al -> 28Al* -> 28Si + e- + nu
Fermi also bombarded Uranium with neutrons, and by analogy with the lighter elements, had some reason to believe that he had made transuranic elements. In fact, he received the Nobel Prize in 1938, for "his demonstration of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons".
It was thought that just like Aluminum can be transmuted to Silicon by slow neutrons, so Uranium could be transmuted into a new element with Z = 93.
n + 238U -> 239U* -> 239 Np + e-+ nu
The name Neptunium for Z = 93, and, in fact, the discovery came only in 1940 and later. One reason for the earlier plausible, but mistaken conclusions, was that it was not clear what the chemical properties of the transuranic elements should be. Thus in the 1930's, it was not easy to test if they had actually been made.
Now, already in 1934, it was suggested by Ida Noddack that a different reaction was occurring, namely nuclear fission into two parts, both quite heavy. A decade earlier, Noddack had been instrumental in the discovery of a missing element, with Z = 75, which she named Rhenium. A few years later, she claimed to have discovered another missing element Z = 43. However, this claim turned out to be of doubtful validity, and the element with Z = 43, was made only later by Segre, who named it Technicium. Thus her scientific credentials were somewhat in question at the time she published the fission hypothesis.Noddack could not give any supporting evidence for her speculative, though remarkably prescient conjecture. Of course, her idea went against the conventional wisdom that no particles heavier than the alpha could be emitted, thus it was not taken seriously by anyone else at the time. Furthermore, Noddack did not follow up on her idea. On the other hand, it must be said that she had no external encouragement whatsoever to do so. (See Rhodes, Pages 230 - 232 for a fuller discussion of this episode) A few years after Fermi, Irene and Frederick Joliot-Curie also bombarded Uranium with neutrons. Like Fermi, they believed to have made transuranics, but they also found some evidence that Lanthanum(Z = 57) was produced in the reaction. But they did not connect this with fission.
Starting in 1936, Hahn and Meitner also looked at the reactions induced by neutrons on Uranium, and investigated the properties of the resulting radioactivities. The results, based, of course, on the assumption that these involved nuclei in the vicinity of Z = 92, (Uranium) were quite inconsistent. For example, they thought that one of the elements made was Radium(Z = 88). What they thought was (to quote Anderson) "radium had been precipated using barium as the carrier element. However, on 19th December 1938 Hahn and Strassman discovered that they could not separate the radium from its barium carrier. Finally they felt compelled to draw the conclusion, that the element did not just behave like barium, it was barium.".
Earlier in 1938, Lise Meitner had left Germany to avoid Nazi persecution, but she and Hahn kept in touch by letter. Soon afterwards Meitner and Frisch came up with the explanation in terms of nuclear fission,which is now taken for granted. It should be pointed out that in 1936/37,Bohr and Kalckar had suggested the possibility of collective excitations involving the nuclear surface. This picture (unlike that of tunnelling through a Coulomb barrier) is consistent with the possibility of nuclear fission. Meitner and Frisch were familiar with the Bohr-Kalckar model, and that made it easier for them to finally overcome the conventional wisdom about emission of heavy particles. The explanation of nuclear fission spread very fast, and early in 1939 this process was soon reproduced in many laboratories. More detailed properties of fission were worked out by Bohr and Wheeler using the nuclear liquid drop model. That was the beginning of the atomic era. It was one of the more dramatic occasions in Physics when science and politics intersected.
It is an interesting what-if exercise to speculate on what might have occurred if Fermi (or anyone else) had taken Noddack's fission hypothesis more seriously. Keep in mind that this was in 1934, and that Mussolini, the dictator of Italy was at the height of his powers.
As a postscript, once the prejudice against emission of particles heavier
than the alpha particle
was broken, it was no great surprise when in the 1980's some clear
evidence for decays involving the emission of 12C nuclei was
established.
I E.G. Segre, "The Discovery of Nuclear Fission" also gives an short summary of the fission story. Emilio Segre was one of the pioneers in nuclear physics of the 1930's.