This series is a fanfic of 'That Time an American was Reincarnated into Another World' by Sp4de and was made with his permission and guidance :>

Tanya von Degurechaff was once feared as the Devil of the Rhine, a mighty mage that every soldier in Europe had at least heard a passing comment of at one point or another. Originally a Japanese salaryman, she was thrust into a world on the verge of war by an angel she refereed to as 'Being X'. Now as his experiment comes to a close, he finds himself the subject of God's judgement. With Being X punished for what he did, Tanya is offered another chance; a world where humanity is at war in a desperate fight for survival. Reborn once more with a far more favorable start, Tanya sets out to purge the world of all that would threaten her new life.

However unknown to her, she was but one person reincarnated from another world. She was not alone, and soon enough the reputations of both the 'Devil of the Rhine' and the 'American' were bound to shake the very foundations of this world.