Lenne had the misfortune to live in an age of turmoil, which indirectly caused the many tragedies that took place in FFX and FFX-2. But what Final Fantasy is without some kind of warfare?
A thousand years ago, Spira prospered through common use of machina, or machinery in modern terms. Contrary to the norm, however, Zanarkand retained a faith-based military system and depended heavily on summoners in time of civil conflicts.
When war broke out between Zanarkand and Bevelle for reasons not explicitly stated in both games, Zanarkand was left devastated as many of its summoners perished by the fearsome sight of machina-based weapons. Refusing to accept a complete defeat, Yu Yevon, then ruler of Zanarkand, created a dream image of his city by using the remainder of his people's life energy (Fayth). He also attempted to surround himself by a great amount of pyreflies as protection, which however gradually consumed his will to live as a human.
As Yu Yevon lost his conscious of reason entirely, his armor became Sin: a destructive creature programmed to annihilate all means of advancement in Spira. It's quite ironic that Yu Yevon's intent to protect Zanarkand from outside forces had actually resulted in its ultimate downfall, thus began a millennium of misery to people of Spira.