The United Nations (also known as the "UN") is an intergovernmental organization whose stated aims are facilitating cooperation in international law, international security, economic development, social progress, human rights, and achievement of world peace.
The UN was founded in 1945 after World War II to replace the League of Nations, to stop wars between countries, and to provide a platform for dialogue. It contains multiple subsidiary organizations to carry out its missions.
Involvement with the Agency
The International Contract Agency has ties to several major governments in the Central Intelligence Agency (via Agent Smith), Federal Bureau of Investigation (prior to The Franchise taking it from them), MI5/MI6, National Security Agency, and Interpol. They are especially friendly with the United Nations; after Agent 47 prevented a terrorist attack at their summit in 2000, they began a tight business relationship.
The Agency later made a rare exception to their rule of not killing former clients, when 47 was requested by the UN to assassinate Sergei Zavorotko for his possession of WMDs. Shortly beforehand, 47 was under strict orders not to kill any UN soldiers while on a mission in Afghanistan. The UN wanted to gain information from 47's target before he was sniped, showing that the Agency does not have a strict allegiance to the UN.