USS Midway: The last stand

How did it come to this? How did the world that had been at peace just 8 years ago, turned into one of the bloodiest conflicts in human history. When Germany attacked Poland in September 1939, the world was dragged into another conflict that the previous war was meant to be the end all of it. But
then, an unknown object entered into the Earth's atmosphere and revealed several humanoid beings with contraptions connected to their backs.

Given the designation as "Siren", humanity shifted their focus and fought the sirens back but were getting nowhere. Slowly, the sirens were beating humanity back and managed to capture almost all of the Pacific and Atlantic. A final battle between humanity and sirens lead up to the Admiral commiting the ultimate sacrifice to save humanity.

In the afterlife, he was given another chance to live in a new world. A world where warships were turned into shipgirls and he must use his knowledge and ability to save them from the same creatures that invaded his Earth.