Jehovah's Witnesses: Just what is or was the controversy regarding the church and the United Nations ?

I understand that JW's condemned the UN for decades as destined to oppose God's kingdom, type of the antichrist or something, and that they are being accused by some of being frauds in this because the JW church alledgedly got involved with the UN is some way. Can you give me a clear understanding of what this situation is? What are the accusations and what is the response?