1939. Humanity was faced with the sudden appearance of a mysterious foe. They called it the Neuroi. In the blink of an eye, the Neuroi successfully invaded most of Europe, destroying many people's homes and conquering nations. To combat the Neuroi humanity developed a new weapon, the flying Striker Units, that could only be used by Witches — girls with magical power. Witches across the world gathered to use these Striker Units against the Neuroi. A unit of these Witches was formed: the Allied Forces' 502nd Joint Fighter Wing, called the Brave Witches.

In September of 1944, thanks to the actions of the Allied Forces, with the 501st Joint Fighter Wing "Strike Witches" at their centre, the Neuroi nest in the skies above the Republic of Gallia was eliminated, ensuring the security of western Europe. The Allied Forces took advantage of this victory to go on the offensive in central and eastern Europe. At their base in Petersburg in the Empire of Orussia, the 502nd Joint Fighter Wing "Brave Witches" prepared to receive orders to deploy... With the hope of mankind resting on their shoulders, the courageous Witches of the 502nd take to the cold skies over eastern Europe.

Source: crunchyroll

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Same setting: Strike Witches (2008)
Same setting: Strike Witches 2
Side story: World Witches Hasshin Shimasu!