The North Atlantic Treaty Organization, also known as NATO, was a multi-national military organization on Earth in the 20th and 21st centuries based in the country of Belgium.
Bases
NATO is believed to have held control of Outpost C9 sometime in the past, as several NATO symbols can be seen throughout the base. There are also NATO symbols on the multiplayer map, Rat's Nest.
Phonetic Alphabet
NATO had developed a phonetic alphabet which was later inherited by the UNSC and currently serves as the official UNSC Phonetic Alphabet.