Complex sentence: When nuclear fission was discovered in 1938, it was soon realized that this might be the mechanism to produce the neutrons for the chain reaction, if the process also produced neutrons, and this was proven in 1939, making the path to nuclear power production evident.
Split into simpler sentences: When nuclear fission was discovered in 1938, it was soon realized that this might be the mechanism to produce the neutrons for a chain reaction, if the process also produced neutrons. This was proven in 1939, making the path to nuclear power production evident.