Lenne had the misfortune to live in a period of great war, which was ultimately responsible for the very many tragedies that took place in both FFX and FFX-2. But what Final Fantasy is without some sort of warfare?
Machina War
The war Lenne became involved in was totally out of personal control, which is why her fate as a summoner was so different from Yuna's. Not all good people would meet happy ends, I suppose.
A thousand years ago, Spira prospered through common use of machina aka. machinery in our terms. Yet Zanarkand remained faith-based and depended much on its summoners to protect their homeland. When war broke out between Zanarkand and Bevelle for reasons not explicitly stated in both games (maybe due to certain people's lust for power?), Zanarkand was left devastated as many of its summoners perished by the fearsome power of machina. Imagine the sad faces of Bevelle leaders when Yu Yevon, old ruler of Zanarkand, created something even more destructive than machina (aka. Sin) and dragged much of Spira to hell with it. It's quite ironic that a war initiated by abusive usage of machines would eventually lead to a millenium of no proper scientific knowledge.