The history fission and fusion is rather short and very convoluted, so a brief description of the two processes is a good place to start. Simply put, "fission" is a nuclear process in which an atom is split (or decays) to form two smaller atoms while releasing a quantity of energy. In the case of "fusion", two smaller atoms are fused together to form a larger atom, also resulting in the release of an even larger quantity of energy. In both cases, the "binding energy" of the end result is higher than that of the initial components. What that implies is greater stability than before. Binding energy refers to the amount of energy held within the bonds of the atom which inhibit the atom from just flying apart into protons, electrons and neutrons. The more energy held in the bonds, the more stable the atom is with respect to its previous incarnation because it requires more mechanical energy to break it apart further. In other words, it becomes a harder nut to crack using either fission or fusion reactions since both require kinetic sub-atomic particles flying around to perpetuate a reaction.
For fission to occur there must be a minimum amount of material present for the fission reaction to be self-perpetuating. If this "critical mass" is struck by a relatively slow moving neutron, not only is fission initiated, but it can become a self-sustaining nuclear reaction. Fusion reactions are a bit more involved. The sun exhibits a fusion reaction, combining lighter hydrogen to heavier elements. On earth a common fusion reaction involves two isotopes of hydrogen in a deuterium-tritium reaction but it takes a bit more than just slamming atoms together. Intense pressure on the sun helps initiate the reaction, while on earth the mixture of deuterium and tritium must be superheated to a plasma state to overcome charge repulsion. The benefit is that fusion releases much more energy than fission. But where does that "energy" come from?
Using Einstein's E=MC^2, it is readily noted that energy and mass are convertible. If you were to take the weights of all the atoms before the nuclear reaction versus after, you would note a small loss of mass. And that's not a rounding error. This "lost" mass is converted into the energy which is released during the nuclear reaction. The histories of fission and fusion are detailed and colorful, but for the sake of brevity, following is an overview of the discovery of fission, followed by fusion.
Prior to the 1930's, it was considered impossible to "split" an atom. Then the neutron was discovered in 1932 by English physicist James Chadworth by bombarding beryllium with alpha particles and the entire sub-atomic world literally went to pieces. Ernest Lawrence's "cyclotron" was still years in the future, but the concept of a nuclear chain-reaction resulting in a nuclear reactor was postulated as early as 1933 by Enrico Fermi and Leo Szilard. Frederic Curie's work in Paris involving the concept of secondary neutrons being released during fission of uranium made the possibility of a chain-reaction feasible. The bombardment of uranium by neutrons was vigorously studied by Fermi and his colleagues in 1934 but it took several more years before the results were properly evaluated. Following Fermi publicizing his research, scientists in Germany began similar experiments. Lise Meitner, an Austrian Jew, was one of these scientists and was later forced to flee to Sweden to escape persecution. But Meitner kept close collaboration by mail with Otto Hahn and research continued.
By 1939, Otto Hahn and Fritz Strassmann had discovered that the neutron bombardment of uranium produced an isotope of barium and published a paper in Germany, which earned Hahn a Nobel Prize. This created a longstanding controversy since Meitner and her nephew Dr. Otto Frisch actually confirmed Hahn's discovery by using a cloud chamber, proving that uranium was indeed split by neutrons as well as providing mathematical proof. While Frisch, Strassmann and Meitner did help prove you could split the atom, they didn't get a chance to split the Nobel Prize. Then it got worst.
Frisch and Meitner also discussed their results with Niels Bohr and before Bohr escaped Nazi-held Denmark by boat, Frisch and Meitner gave him their calculations. The number of nuclear fission players increased again while Bohr was escaping and discussed Meitner and Frisch's paper with Leon Rosenfeld. Bohr had promised to keep their paper and discover secret until it could be published, but Bohr had failed to extract the same promise from Rosenfeld. Loose lips may sink ships, but in this case the boat eventually found safe harbor in the U.S. And no sooner than Rosenfeld arrived at Princeton University, he began telling everyone about Frisch and Meitner's work until word spread all the way to Fermi at Columbia. Needless to say, the genie was out of the bottle and before long several experiments to confirm fission had been initiated.
U.S. interest in nuclear reactions came partially out of scientific curiosity, partially from fear that Germany was working on a weaponizing nuclear energy. After Frisch moved to England, he continued his work with Rudolf Peierls and demonstrated that fission of uranium had the potential to create a destructive weapon of unimaginable magnitude. . Physicists presented other theories, including the concept of using a purified isotope of uranium-235, which would greatly reduce the mass of material required to create and perpetuate a reaction. The concepts of gaseous diffusion separation, electromagnetic separation and use of plutonium as a substitute of uranium rapidly followed. Initially shunned, Frisch was finally permitted to work on the Manhattan Project since it was his earlier work that made such a project possible.
It is worth noting that Fermi's first attempt to make a nuclear pile failed, thus revealing that though the theory may be sound, the actual accomplishment of a nuclear chain-reaction was not trivial. The amount of uranium available in those days was incredibly small, not to mention issues of purity. In spite of those obstacles, Fermi and his colleagues at the University of Chicago eventually achieved the first controlled, self-sustaining nuclear chain reaction in December 1942. Needless to say, more scientists eventually succeeded in creating a sustained nuclear reaction and before long, The Bomb was born. And the restis history.
The second way to create a nuclear reaction is through fusion. The first fusion experiments were conducted in Cambridge, UK during the 1930's but initial results were less than promising and the concept of fusion was mocked and abandoned. After World War II, fusion was revisited, especially after the success of the Manhattan Project. Though the initial expressed purpose for the research was peaceful uses of fusion energy, the focus quickly turned to its destructive capability, where results could be counted on more quickly.
A German born physicist named Hans Bethe, recruited by Robert Oppenheimer to join the Manhattan Project, accepted at the urgings of collaborative research colleague Edward Teller. Ironically, in the late 1930's Bethe had written a theoretical paper casting doubt on fission. However, also in the late 1930's, Bethe (together with Peierls) first recognized that fusion of hydrogen nuclei to form deuterium was an energy generating mechanism utilized by stars. He later went on to win a Nobel Prize for extensions of that early work. Now at Los Alamos, he made advances in the determination of critical mass while head of the Theoretical Division. Unfortunately, Teller also wanted that position, and their relationship would increasingly approach critical mass as time went on.
After the war, there was a push for hydrogen bomb research; Of course, the first primary projects involving fusion were military applications since early fusion bombs could theoretically yield 500 times the amount of energy as a fission bomb. In fact, a fission bomb could be used to set off the fusion bomb, providing both the pressure and temperature which would be used to create fusion reactions. Bethe himself did not believe a hydrogen bomb was possible, or at the very least, he hoped it was not. He was not alone. In 1954, Bethe testified on behalf of Oppenheimer during a high-profile security clearance hearing.
The security hearing was in part as a result of having discovered espionage at Los Alamos. Most notable was British physicist Klaus Fuchs who had offered his services to the Soviet Union in the early 40's. After Fuchs was reassigned to bomb research at Los Alamos...in Bethe's Theoretical Division no less...he passed on information regarding weapons designs until he was caught and confessed in 1950. Suspicion of Oppenheimer came from several quarters and not all of them rational. Bethe contended Oppenheimer's opposition to the development of the hydrogen bomb in the late 1940s had not hindered its development. Bethe tried to convince Teller not to testify, but Teller did anyway, resulting in the revocation of Oppenheimer's security clearance. Their collaborative honeymoon was over and as time went on, Teller became known as the father of the hydrogen bomb, with Bethe as the self-professed "midwife".
Unlike the Manhattan Project, most interest in fusion was primarily for peaceful uses with the expressed desire to control the release of nuclear energy. A false claim of success in 1951 by scientists in Argentina acted as a catalyst to many research groups. Among those research groups include those headed by Sir George Thomson at Imperial College and Peter Thonemann at Oxford. Sir Thomson actually patented a fusion reactor, but the first large-scale experimental fusion reactor was developed in the 40's and called ZETA (Zero Energy Toroidal Assembly). It was operational between 1954 and 1958 and results were positive.
In the US, Lyman Spitzer at Princeton was developing a magnetic confinement device called a stellarator, while Edward Teller was working on the hydrogen bomb over at Lawrence Livermore Laboratory. The former Soviet Union was also performing significant fusion research using the theoretical work of I. E. Tamm and A.D. Sakharov in developing the tokamak (a donut shaped magnetic field for containing plasma). However, civilian applications were still lagging. While it took about a decade for fission to go from solely military applications to civilian applications, in the past 50 years fusion has not seen much civilian cross-over.
In 1978 the JET project in Europe was initiated, finally coming on line in 1983. By 1991, JET produced the world's first significant amount of power from a controlled nuclear fusion reaction. In 1993, Princeton's Tokamak Fusion Test Reactor (TFTR) device, using a deuterium and tritium fueled plasma, produced 10 MW of power. By 1997 JET seized the world record for fusion power by producing 16 MW of power. In 2005, construction of another experimental reactor (ITER) was announced, which is designed to produce several times more fusion power.
The fusing of two atoms together takes not only incredible pressure, but intense temperatures, which makes creating and controlling this reaction very difficult on earth (while not so difficult on the sun). One advantage of fusion over fission is that the by-products are much less hazardous. While wroth with difficulty, many still believe that fusion is the energy of the future. As a final note, the theoretical "cold fusion" would greatly alleviate the environmental difficulties associated with regular hot fusion. That is, once cold fusion results have been definitively reproduced. But, that's another story.