The United Nations is a real-world international organization created for the purpose of fostering positive relationships between nations. One of the primary functions of the UN is to establish and fund other organizational bodies to solve various issues and crisis, a function which lead to the creation of various pivotal organizations in the Evangelion franchise.
Following the global ecological disaster and subsequent political upheaval caused by the Second Impact, internal reorganizations lead to a heavy militarization of the UN, now actively deploying military forces across the globe. Additionally, the militias of both the Western and Eastern power blocs were put under the UN's direct command after Second Impact further increasing their military might.
During these reorganizations, NERV is established as a special agency under the UN's control to defend against an inbound and unknown threat; the angels. SEELE, more specifically the Human Instrumentality Committee is established as an overseer for NERV, however in reality, NERV has very little oversight beyond matters of funding as it is revealed that their overseers are actually a powerful cabal secretly running the UN and the world from the shadows.
After the events of the second impact, special agency NERV is established under the supervision of the human instrumentality committee to defend against the threat of angels. Due to the importance of their work, NERV is given seemingly limitless amounts of funding and resources to develop Project-E during the 2000's and early 2010's.
Based on a newspaper from 2002 announcing "UN moves headquarters to Japan in 5 years: Candidate City is Tokyo-2." it is likely that at some point in 2007 the UN relocated their headquarters from New York City, New York to Tokyo-2, Japan.
In 2014, Sachiel arrived as the first angel to launch an attack on Tokyo-3. In an attempt to stop the angel, United Nations Type-74 Nana-yon tanks were dispatched. After the failure of conventional weaponry to show any effect on the angel, command of the operation was handed from the Japanese Strategic Self Defense Force to NERV resulting in the first deployment of Evangelion Unit-01.
Due to the increasing frequency of angel encounters, NERV requests Evangelion Unit-02 and its pilot Asuka Langley Sohryu be transferred from the Germany base to the Tokyo-3 base and commission the UN Pacific Fleet in order to ensure they're transported safely. Dismayed by the increasing irrelevance of the once great fleet, the commanding officer openly lamented that the UN's forces had become a "delivery service," for NERV, refusing to sign over the Evangelion unit to NERV until after its arrival in Japan as retaliation. Unbeknownst to the fleet, the main purpose of the mission was not in fact to transport the Evangelion unit, instead being to ensure the safe transportation of the embryonic Adam with Unit-02 being there to provide added protection in the event of an angel attack and to conceal the existence of the embryo.
As was expected, during this mission the angel Gaghiel attacked the fleet in an attempt to capture Adam leading to the unauthorized dispatching of Unit-02. In order to destroy the angel, many naval ships as well as various aircraft stationed on them were destroyed. It is later stated that one third of the UN Pacific naval fleet was destroyed during the confrontation.
Following the events of the Second Impact special agency NERV is created in order to fight off the future threat of the Angels through the use of the Evangelion. Due to fears of any one nation possessing an army of the most dangerous weapons to ever exist, the Vatican Treaty is established to limit the number of active units any nation can control to 3. In order to enforce these rules, the IPEA is established and granted the authority to requisition units from any nation which surpasses the limit.
Due to a lack of nations following the events of third impact, the United Nations ceases to exist. The Vatican treaty is referenced during the Paris Assault Mission as after the disbanding of the United Nations, NERV began to mass produce Evangelion units for the express purpose of weaponizing them to form an unstoppable army.
According to Kōzō Fuyutsuki, the UN was divided in an internal conflict between two divisions, and the one who had part in the militaries of various countries, decided to fix the situation by splitting the NERV into various different branches in name of restoring order. 
Equipment Used (NGE)
- Evangelion Unit-02 - While not deployed by the United Nations, it was under their authority when it launched to engage Gaghiel
- Type-74 Nana-yon Tank
- Iowa-class battleships
- Kirov-class battlecruiser
- Admiral Kuznetsov-class aircraft carrier
- Kiev-class carrier Novorossiysk
- Hatsuyuki-class destroyers
- Oliver Hazard Perry class frigates
- Alreigh Burke/Kongo class guided missile destroyers
- Yak-38 VTOL
- Sukhoi Su-33 Flanker-D aircraft
|Neon Genesis Evangelion||NERV • Gehirn • SEELE • Human Instrumentality Committee • Marduk Institute|
|Rebuild of Evangelion||NERV • Euro-NERV • SEELE • WILLE • KREDIT • JMEPRO • IPEA|
|Neon Genesis Evangelion: ANIMA||NERV Japan • NERV U.S. • NERV Germany|
|Other||Japanese Strategic Self Defense Force • United Nations • Global Unknown Event Keepers|
- Neon Genesis Evangelion: ANIMA Volume 2, chapter 58