5 was showered with radioactive fallout from the U.S. military's 15-megaton "Castle Bravo" hydrogen bomb test at nearby Bikini Atoll.
[1] In response to these events, a number of Japanese civic groups banded together to form a "National Council for a Petition Drive to Ban Atomic and Hydrogen Bombs" (Gensuibaku Kinshi Shomei Undō Zenkoku Kyōgikai), which managed to accumulate a total of 30 million signatures by August 1955,[1] an impressive figure given that Japan's total population was around 90 million at the time.
[3] The organization touted itself as apolitical in order to secure as broad a base of support as possible, and thus drew support from both left-leaning and conservative politicians alike[1][3] In the later half of the 1950s, Gensuikyō became a vocal leader in the burgeoning worldwide anti-nuclear movement, deriving moral authority from Japan's unique status as the only nation to have been attacked with nuclear weapons.
Gensuikyō played an active and enthusiastic role in carrying out the large-scale 1960 Anpo protests against revision of the U.S.-Japan Security Treaty, as many of its members believed that the treaty would place Japan in danger of future nuclear attack by having Japan side with the United States in the Cold War.
These disagreements led to a schism in Gensuikyō,[5] with the JSP-affiliated groups splitting off to form a rival organization bearing the very similar name "Japan People's Council against Atomic and Hydrogen Bombs" (原水爆禁止日本国民会議, Gensuibaku Kinshi Nihon Kokumin Kaigi)), usually abbreviated Gensuikin.