Azur Lane is set in an alternate World War II timeline where Earth's oceans are invaded by a mysterious alien force known as the Sirens. These beings possess advanced technology far beyond humanity's capabilities, causing nations to lose over 90% of their maritime control. In response, four major naval powers—the Eagle Union (U.S.), Royal Navy (UK), Iron Blood (Germany), and Sakura Empire (Japan)—form the Azur Lane alliance. They use Wisdom Cubes to create shipgirls (Kansen), humanoid embodiments of warships, to fight back.
Initially successful, the alliance fractures when the Iron Blood and Sakura Empire defect to form the Crimson Axis (Red Axis). They adopt Siren technology to enhance their power, believing it is the only way to ensure humanity's survival. This ideological split reignites global conflict.