From Halopedia, the Halo wiki
Unified Earth Government
New Jerusalem was the capital city of the planet New Jerusalem, a human Outer Colony in the Cygnus system governed by the Unified Earth Government. As is the case on several human colonies, the city shares its name with the planet as the first population hub was once synonymous with the planetary colony itself.
The city was likely destroyed in 2552, during the Battle of New Jerusalem when the planet was glassed by the Covenant.