The United Nations (UN) is the intergovernmental organization that has the objective to maintain the peace and security in the world. It was created to promote international cooperation after the end of World War II, to correct the failures of the League of Nations created after World War I, and with the development of nuclear weapons, prevent a potential World War III.
The Rescue Aid Society is an international mouse organization composed of United Nations delegates. It is located in the walls of the United Nations headquarters.
After Wanda Maximoff accidentally destroys an entire building, causing several deaths during a mission of the Avengers in Lagos, the United Nations proposes an accord in which the superheroes would have to submit to the organization's authority in order to choose when and where to act during their next missions, the Sokovia Accords. Part of the heroes refuse to sign the Accords, preferring to continue without being under the government's restrictive conditions to their actions. As result, the Accords imposed by the UN result in the heroes being split into two groups: the ones that agree with the UN about the Accords and the ones who don't.
Later, T'Chaka, the King of Wakanda goes to the Vienna International Centre, a facility of the UN, to speak in favor of the Accords. However, T'Chaka is killed during an explosion that destroys the entire place. Bucky Barnes is framed as the culprit and is hunted for it. Captain America, knowing that the accusation was false, protects and hides his friend with the help of some of his allies as Sam Wilson and Sharon Carter, who also become wanted by the government who sees them as criminals too. Tony Stark presses Steve Rogers to just sign the Accords and hand Bucky to the government, but he refuses.
Steve then recruits more heroes to help him and leads the group to the Leipzig-Halle Airport to travel to Siberia while looking for the real murder of the King of Wakanda, but is confronted by Tony Stark who also has a group of heroes supporting him. After the two groups fought, all the heroes in Captain America's group were seen as criminals by the government. Some of them are immediately arrested, but later Steve breaks most of them out of prison, with only Scott Lang and Hawkeye having to deal with a house arrest, while all the others become fugitives. On the other hand, the members of Tony Stark's group sign into the Accords and are allowed to remain free.
After the Civil War, the US government continues to hunt down the superheroes who didn't sign the United Nations' Sokovia Accords until Thanos' invasion of Earth.
During the end of the film, the new King of Wakanda, T'Challa, goes to the Vienna International Centre, to tell to the United Nations the truth about Wakanda, which was a country hidden from the world for a long time. He then announces that, from that moment on, Wakanda won't hide anymore and will share its advanced technology with the rest of the world.
In the third episode, in an alternate timeline set before the events of The Avengers film, all the cadidates of the Avengers Iniciative are killed. After Loki helped Nick Fury to capture the murderer, Hank Pym, he travels to the United Nations headquarters along with his Asgardian army. There, Loki announces his rise to power, declaring that he managed to unite all the nations of the world in a single day, to serve under his command.