Bikini Atoll is probably most famous for two things. Trivially, the atoll gave its name to the popular bathing suit. More importantly, the U.S. conducted early post-war nuclear testing at Bikini between 1946 and 1958.
Under the terms of the Peace of Cairo in 1944, Japan retained possession of Bikini Atoll. In 1965, decades after the first explosive-metal bombs were used, Japan was able to successfully detonate their first device at Bikini and demand diplomatic relations with the Race. As the Race had only recently removed Germany as a threat, Japan's ascension was not welcome news.