From Halopedia, the Halo wiki
The North Atlantic Treaty Organization, also known as NATO, was a multi-national military organization on Earth in the 20th and 21st centuries.
Prior to 2552, NATO is believed to have held control of Outpost C9 and Crow's Nest in Africa, as several NATO symbols can be seen throughout the two bases.
NATO had developed a phonetic alphabet which was later inherited by the UNSC and currently serves as the official UNSC Phonetic Alphabet.
The UNSC's M118 7.62x51mm FMJ-AP round, used in the MA5 series and other weapons, is based on the 7.62x51mm NATO round.
The NATO symbol at Outpost C9.
The NATO symbol on Rat's Nest.