During the war Derpy was sent to the frontline as a part of the recon squad. She took part in the most dangerous and desperate battles and always returned home, usually as a solo survivor. It was soon discovered that her enemies were killed in most horrible and gruesome way.
Derpy's response? "I don't know what went wrong".
Soon pony started to really FEAR her, and she started to climb military ranks quickly, eventually gaining the rank of Corporal(?).
After that she was unofficially called as a Pale Plague, the one of War Abominations. (Including Lyra "Steal Hurricane" Hearthstring and Pinkie "Diane" Pie)
Probably at this point her rise to the General rank and the throne was inevitable. And I think she would gladly give everything back just to return Luna