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

What would an ultra-rational Japanese salaryman do if he was reborn as a girl in a world where magic was real? Obviously, become the Empire's most feared magic soldier, while climbing the army career ladder at the ripe old age of nine. The only things standing in Tanya Degurechaff's way are a world war and maybe a supernatural being or two. But Tanya isn't about to let artillery shells or deadly spells stop her when a cushy desk job is calling!