They called it the Greatest War. Land, sea, and air all embroiled in savage combat all over the world for over half a decade. Through sacrifice and fighting, that conflict drew itself to an eventual close and the world knew tense peace. The efforts, the courage, the sacrifices, all forgotten as decades wore on. Forgotten, but not gone.
Taking offense to how their history had begun to fade from human memory, the great warships of history rose up into what became known as the Fleet of the Abyss. Assuming humanoid forms and carrying their vengeance on humanity, these ghostly warships threw themselves against humankind's defenses. Though they could be held back, the toll forced many nations to rely on the few remaining superpowers for protection and stretched resources thin around the world. Any nation that had territory on the coastline wanted protection, not all could be protected.
Not all of the arisen Abyssal Ships appreciated the mission that their leaders had laid out for them, however. Many began to doubt how honorable their mission really was. This divided the Abyssal Fleet until, in a schism between the Battleships, resulted in the complete separation of the Abyssal Fleet into two separate units. The first, the original Abyssal Navy, remained under the leadership of the Battleship Princess and her subservient Princesses. The second, the splinter navy, began calling themselves Fleet Daughters. United under the joint command of Nagato and Musashi, these warships took up arms against the Abyssal Navy by shedding their corrupted exteriors and siding with Humanity.
These Daughters stood against their former brethren to protect the innocent, unified under the belief that the current generations should not be held accountable for the actions of their ancestors.
Thus began a new war, a naval war, between the offended ghosts of the forgotten past and the progressive spirits of a unified future.