Why existential threats create unlikely coalitions

Is Azur Lane's idea that hostile powers must unite against the Sirens historically reasonable?

Yes; Churchill and Stalin demonstrate that states with profound mutual hostility can cooperate temporarily when they face a more immediate existential enemy.

Azur Lane's Siren wars raise the question of whether hostile powers can cooperate when defeat threatens them all. Churchill's Britain and Stalin's Soviet Union supply a historical comparison: their hostility to Hitler took priority over their hostility to one another. Cooperation in that emergency did not require the partners to like each other. The fictional comparison varies by timeline, however; the panel describes both worlds where the Sirens drive cooperation and worlds where the characters fight without that common enemy.