In Halo 2, the human city where the opening battle takes place is New Mombasa.
New Mombasa is a major city in Kenya on Earth. The game begins during the Covenant assault on Earth, with Master Chief aboard the orbital defense station Cairo Station before the battle moves to the city below. The urban campaign includes streets, bridges, waterfront areas, and the city’s enormous space elevator.
The city is important because the Covenant fleet is pursuing the Prophet of Regret, whose flagship enters slipspace from New Mombasa. That jump leaves behind a destructive shockwave and sets up the next stage of the story. The later mission “Metropolis” takes place in and around the city as UNSC forces try to repel the invasion.
New Mombasa is often confused with Old Mombasa, a related settlement in the same broader region. Halo 2’s principal gameplay location is New Mombasa, while Halo 3: ODST revisits the city after the events of Halo 2.