The Saga of Tanya the Evil (Youjo Senki)

The Saga of Tanya the Evil synopsis: Tanya Degurechaff is a young soldier known for her predatory brutality and incredible tactical ability, earning her the moniker "Devil of the Rhine." Underneath her innocent exterior, however, lurks the soul of a man who challenged Being X, the self-proclaimed God, to a battle of wits, resulting in his reincarnation as a young girl into a realm of magical warfare. Tanya, hellbent on resistance, intends to rise through the ranks of her country's military as it progressively descends into world war, with only Being X standing in her way of resuming the quiet life she once knew. However, her foresight and battle initiative have an unanticipated consequence: driving the vast Empire to become one of the most powerful nations in human history. (Source: Mangamonster.net)