It is interesting that even in the early days of the Manhattan Project, Edward Teller suggested the possibility of a Fusion Bomb. It was decided that would be too complex, so the initial project went with Fission Bombs. The Gadget (Trinity Test), Little Boy (Hiroshima), and Fat Man (Nagasaki) were all Fission Bombs. In Fission Atomic Bombs, energy is released by splitting atoms. In Fusion Bombs even more energy is released by fusing two smaller atoms into one larger atom. Fission Bombs are typically measured in Kilotons. One Kiloton is equivalent to 1 thousand tons of TNT. Fusion Bombs are measured in Megatons or millions of tons of TNT.
With the successful completion of World War II, Los Alamos and the other National Laboratories quickly began working on Fusion Bombs. These bombs are also called H Bombs, or Thermonuclear Bombs. The efforts were quickly successful, but the initial Hydrogen Bombs were very large. Efforts then got underway to make smaller bombs, and they were able to make small tactical nuclear warheads that could be fired as an artillery shell. The picture above shows a thermonuclear bomb fired from a cannon.