New York City
From Halopedia, the Halo wiki
New York City is a major city on Earth located in the state of New York in the United States. It is a very well known city on Earth for its media and cultural diversity. One of its current day nicknames is the greatest city on Earth. [1]
United Nations
It is possible that the United Nations Space Commands supreme headquarters is located in New York City, where the current day United Nations HQ is located. It is also probable that it is one of the most heavily defended cities on Earth.