Short answers, pulled from the story.
Nuclear fusion is a reaction in which two or more atomic nuclei combine to form a larger nucleus. The difference in mass between the reactants and products is released as energy, due to the difference in nuclear binding energy before and after the reaction.
American chemist William Draper Harkins first proposed the concept of nuclear fusion in 1915. Arthur Eddington then correctly predicted in 1920 that fusion of hydrogen into helium was the primary source of stellar energy.
The first experimental demonstration of fusion is widely considered to be the May 1934 publication by Mark Oliphant, Paul Harteck, and Ernest Rutherford at the Cavendish Laboratory, which described an intentional deuterium fusion experiment and led to the discovery of both tritium and helium-3.
The first artificial thermonuclear fusion reaction occurred during the 1951 US Greenhouse George nuclear test, using a small amount of deuterium-tritium gas and producing a yield of 225 kt, fifteen times that of Little Boy. The first true two-stage thermonuclear device was the 1952 Ivy Mike test, which yielded over 10 Mt.
The US National Ignition Facility announced on the 13th of December 2022 that on the 5th of December 2022 it had achieved break-even fusion, delivering 2.05 MJ to the target and receiving 3.15 MJ of fusion energy output. However, the total power supplied to the experimental cell is hundreds of times larger than the power delivered to the target, so system-level net energy gain has not yet been achieved.
ITER is currently expected to initiate plasma experiments in 2034, but full deuterium-tritium fusion is not expected to begin until 2039.