1954

On March 1, 1954, the U.S. tested the first H-bomb, Castle Bravo, on the Bikini Atoll in the Marshall Islands. Having not warned Japan, a fishing boat called the Daigo Fukuryu Maru (Lucky Dragon No: 5) was sailing nearby and was hit by the fallout, resulting in the death of the ship's radioman. Most fish caught around Japan for the next few months were contaminated by drifting fallout and could not be eaten, causing a severe downfall in the fishing industry.
Tomoyuki Tanaka, a producer at Toho Studios, developed an idea for a movie about a giant monster created by an H-bomb test that attacks Japan.
The first Godzilla is released on November 3 in Japan.

In the Kaiju Multiverse

Godzilla, a giant 50 meter kaiju created by the radioactivity from an H-bomb test, attacks Japan and completely destroys the country's capital city of Tokyo. The beast is finally killed by the Oxygen Destroyer, a device invented by Daisuke Serizawa. The doctor, however, sacrificed himself and died with the King of the Monsters in order to insure that his device would never be used again. (Godzilla)
On Lagos Island, three man-made animals called Dorats are exposed to a nuclear blast and merge into one, giant monster: King Ghidorah. (Godzilla vs. King Ghidorah)
In March, the two Shinomura fuse into one and attack Moansta Island. The Moansta Islanders contact Ishiro Serizawa's father, who contacts Monarch and Godzilla battles it and uses his atomic breath on it, splitting Shinomura into two. Godzilla kills one of the halves but the other escapes and Godzilla goes after it. The U.S. army decides to intercept and try to kill them at Bikini Atoll. The Castle Bravo nuclear "test" is conducted, killing the Shinomura. Godzilla, however, survived. (Godzilla: Awakening/Godzilla)