Hostilities arose, leading to warfare occurring at various points, such as on Earth's Moon, as shown in Alita's flashback, eventually leading to a devastation of the two capitals, with an unknown virus being deployed by the URM on Earth and reverse-terraforming nanites being released on Mars. Sometime before the turn of the 23rd Century, Mars and other related colonies had become independent of Earth and The Imperium, as the URM, or United Republic of Mars.

With the dark age that followed, most of the details about the Great War are left unknown, except that Zalem (Tiphares) survived as the sole floating city on Earth, along with the destruction of the colonial empire, isolation of Earth from space, and Alita being left as an apparent sole survivor of the war, aside from Desty Nova. between the Earth-led Colonial Empire and the United Republic of Mars, and resulted in the death of most life on Mars along with the loss of colonies and cutting off of Earth from space Travel, along with human civilization becoming consolidated heavily in Iron City, which in turn survived on a symbiotic relationship with Zalem above.

In the continuity of the live action film, Alita: Battle Angel, The Fall refers to the Great War between the Imperium and the United Republic of Mars, its aftermath, and the beginning of the Dark Age that followed.