If I recall, the Soviet Union petitioned to have all member soviet nations participate individually in the UN, this was not allowed. Why then are individual EU nations allowed to retain their memberships?
Actually Ukraine SSR and Belarus SSR were allowed to participate individually in the UN. Stalin actually did push each Soviet Republic to be included in the UN general assembly as its own sovereign nation. In response however, the US demanded that each of the 48 US states also be admitted to the UN general assembly as its own sovereign nation. The compromise reached was that two of the SSRs to be allowed to be admitted to the general assembly "independently".