Which of these is correct?
The Union of Concerned Scientists's main goal is to prevent nuclear war.
The Union of Concerned Scientists' main goal is to prevent nuclear war.
The noun phrase that is taking the possessive ("Union of Concerned Scientists") is singular, which would suggest adding
's. But the actual word on which the apostrophe is appended ("Scientists") is plural, which would suggest only adding an apostrophe.
Wikipedia gives the former construction as an example (in the subsection "Basic rule (compound nouns)"), but the article does not specifically address this issue in its main text.