When Princess Luna is blindsided by a sudden appearance by Chrysalis, she becomes hypnotized by spell cast by the Changeling Queen. While Equestria lives another day undisturbed, Luna is completely engulfed by the spell, and falls under the Queen's power.
Celestia is unknowingly ruling on her own, and the elements are sleeping undisturbed, unaware of the dangers ahead. The dawn of a new age in Equestria also gives birth to higher technologies, medicines, spells, machines and most importantly, weapons.