It was the year 1939 when the Neuroi appeared. Nobody knows where they came from or what their ultimate agenda is, but the fact remains that their attacks drove people out of their towns and cities.

In order to take arms against them, humanity develops a new anti-Neuroi weapon called the Striker Unit. Using the power of magic to fight against the monsters, this new device enhances and amplifies the power of female magic-wielders. To use this device, young witches from all over the world have been brought together to form an elite task force unit called the 501st Joint Fighter Wing, commonly known as the Strike Witches.

Source: ANN

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Side story: Strike Witches
Sequel: Strike Witches 2
Same setting: Brave Witches
Same setting: Renmei Kuugun Koukuu Mahou Ongakutai Luminous Witches
Side story: Strike Witches: 501 Butai Hasshin Shimasu!