Millimos Saga: Battei Ouji no Tensei Senki

Synopsis:

After dying in a train accident, he is reborn in another world as the youngest prince of a small, vulnerable mountain kingdom. Expecting a quiet and carefree life, he instead trains himself in magic and holy arts, eventually rising to the rank of military marshal. Possessing two normally incompatible powers, Milimos is gradually drawn into a full-scale national war—despite having no desire to fight.