The year is 1944. The nations of the world have been at war for several years with the mysterious invading aliens known as the Neuroi. Karlsland, Orussia, Gallia, and most of the rest of Europe have all fallen or been shattered by the invading force. Britannia is all that stands between the aliens and total domination of Europe.
Enter the 501st Joint Fighter Wing. The only force capable of standing up to the advanced technology of the Neuroi, the 501st specializes in combining magic and technology. Using custom built Striker units, these Strike Witches fight to liberate Europe.
As of 1944, Gallia has been liberated. The bid by a corrupt Britannian general has been halted. The war has not ended, but the efforts of the Strike Witches have given humanity a big step towards victory in the European mainland.
This places Strike Witches-1 at right after episode 13, at the start of season 2. The only substantial difference from show canon is that modern regulations require skirts or pants as part of a woman's uniform.