Key discoveries leading to the nuclear-fission reactor:
1896: France. Antoine Henri Becquerel discovers the existence of radioactive elements.
1905: Switzerland. Albert Einstein announces the equivalence of mass and energy.
1912: England. Ernest Rutherford discovers the atomic nucleus.
1932: England. James Chadwick discovers the neutron - an electrically neutral particle. Neutrons and positively charged protons make up atomic nuclei.
1934: Italy. Enrico Fermi shows that neutrons can disintegrate atoms of many elements, including uranium.
1939: Germany. Otto Hahn and Fritz Strassman discover uranium fission.
1942: United States. Enrico Fermi and his team demonstrate the first sustained and controlled uranium fission chain reaction.
Dr. Fermi later noted that, until this final step, "No one had any idea or intent ... of contributing to a major industrial or military development."