What events took place first of a uranium chain reaction?

Nuclear fission emits a lot of energy on the scale of tiny nuclear nuclei. If fission was limited to a few nuclei of uranium-235 or plutonium_239 the energy released would still remain very small at our scale. On the other hand, if the reaction involves a very large number of nuclei, it is on our scale that this large release of energy will manifest itself.

It is the chain reaction process that is used in reactors and nuclear weapons to generate a large number of fissions. In a reactor, the propagation of fissions takes place in a controlled manner, in a nuclear weapon in an uncontrolled, explosive manner.

The products of nuclear fission are usually accompanied by two to three secondary, free neutrons which can be absorbed later by other fissile nuclei. These absorptions in turn lead to further fissions, then to the production of other neutrons etc ..., a process commonly called chain reaction.