We have been told that Luna rebelled because of her sorrow, but the story isn't so simple as it seems. The Royal Sisters were in war even before coming of the Nightmare Moon. What was the reason behind it? Ideological differences.
A thousand year later, after Luna is reformed, someone starts gathering forces, using old contracts and ancient magic for his own purpouses ,the plans seems perfect, expect for one flaw being ; Twilight Sparkle.