The Saga of Tanya the Evil, Vol. 4 (light novel) by Carlo Zen

A devil wanders the battlefield in the guise of an adorable young child and her name is Tanya Degurechaff! After returning from the sandy southlands, Tanya receives an incredibly suspicious order from headquarters to embark on a training exercise. In reality, command has sent her on a covert mission to initiate a border conflict with the Federation. Soon the Empire finds itself embroiled in another fight it cannot back down from, even if it means making the entire world their enemies!