"Gojira" was a Japanese horror film (it would not be considered a "monster movie" until it reached American shores) that depicted a large radioactive animal (believed by scientist characters to have lived sometime in the intermediate stage between the prehistoric era and current times) that attacks and sinks several fishing vessels and finally ends with his raiding Tokyo twice over the course of a few days, resulting in the city's destruction and wide-scal deaths. His rampage is clearly meant to reflect the atomic bombings of Hiroshima and Nagasaki (though no dirrect political connection is ever made), though most people do not know how the concept for the movie was originally created.
"Gojira" was inspired by America's "King Kong," which lead "Gojira" creator Tomoyuki Tanaka to want to create a horrow film using a monster.
But the reason a radioactive beast that lays siege to Tokyo was used was becuase of an American hydrogen bomb test designated "Castle-Bravo" whose fallout contaminated a Japanese fishing trawler, resulting in some of the crew's deaths and a national recall of fish products because of radiated tuna aboard the vessel that was served into the market unknowingly. An H-Bomb setup is also used to explain Godzilla's strange powers and hideously deformed physik.
"Castle-Bravo" has become the calling card of anti-nuclear activists ever since the incident involving the trawler, and Godzilla has seen more films than any other character.