Azur Lane The Animation: Anata No Aoki Kouro Ni Shukufuku Wo 1

Set on a planet covered 71% by oceans, enemies from another world called Sirens suddenly appeared. In response to this powerful new enemy, the four major powers of the world—the Eagle Union, the Royal Navy, the Sakura Empire, and the Iron Blood—banded together to form the global union Azur Lane to combat the Sirens using anthropomorphized battleship girls called shipgirls. But even since the threat was eliminated, rifts have begun to form between the factions. Soon, tensions have grown, and the Iron Blood and Sakura Empire break off from Azur Lane to form their own coalition. Now known as the Red Axis, they formally declare war on Azur Lane and mount a surprise attack!
Notice: This book is written in Japanese.