Diamond City was once a strong civil establishment that at a point was managed by Mayor Wario. The city managed to evade the first campaign started by the Lord of Darkness when his army was strong. However, their second campaign went right through the city and it was brutally destroyed with the attack. The mayor ran away upon the attack, with a lot of gold coins in his hands. Major numbers of death occurred within the city during this time. There were some who got away. Kat and Ana were among this low number. Evidence exists that Mona survived as well. Certain other individuals have been hinted at surviving as well. The buildings remain in their same state, even after an occupation and another one. Kat and Ana long for the day that they can rebuild the city.
Cody and Kat along with Ana worked together to help restore the city. The technology Cody deployed was a major help. With the city restored, those who fled gradually returning and the city was able to bring itself back to the lively state.