Story
High above the blood- and mud-soaked trenches, a young girl pits herself against army mages in high stakes aerial duels with bullets, spells, and bayonets. Her name is Tanya Degurechaff and she is the Devil of the Rhine, one of the greatest soldiers the Empire has ever seen! But inside her mind lives a ruthless, calculating ex-salaryman who enjoyed a peaceful life in Japan until he woke up in a war-torn world.
Synopsis via AniList (English).
Preview
From the anime adaptation: Saga of Tanya the Evil.