If she decided to take out an army she would be endangering innocent brain-washed ponies and may cause harm to more of her own. Hr magic is too powerful as seen when battling Nightmare Moon or like any other Alicorn such as Twilight while battling Tirek. The point is Alicorn Magic is too strong and will cause to much harm to the lives of ponies and the terrain. This would explain why she and Luna do not do a lot and have their 'heros' - the mane six to care for the danger in alot safer way. Luna's and Celestia's jobs are merely to raise the night and day and fight when absolutely necessary.



As for Nightmare Moon being banished or killed, it is very possible. I doubt Celestia would never want to kill Luna so she must hae been concealed again.



Damn this comment is late but oh well.