The Saga of Tanya the Evil, Vol. 1 (manga)

When the average Japanese salaryman is suddenly thrown into in a world wracked with warfare and hardship by a supernatural power, they might first think to hide or run away. But not Tanya Degurechaff. A calculating and utilitarian man has been reborn as a child soldier. This young girl will do anything to rise in rank and find a way to live a life of comfort, and woe to any king, country, or god who stands in her way.