Before Sin arrived, a war between Bevelle and Zanarkand broke out one thousand years before Final Fantasy X. It was called the Machina War Bevelle used machina to fight, however, because Zanarkand were against the usage of machina, they used summoners instead. Both machina and summoners are equally powerful, except Zanarkand was at a great disadvantage. They were outnumbered because there was shortage of summoners. Thus, there was a very small chance of summoners coming home alive.
As I explained before, Lenne was a summoner as well as a songstress. She knew dead well that she wouldn't survive, but she knew exactly what she had to do. She would defend her home and beloved people, even at the cost of her own life.
The outcome of the war was totally unexpected. It didn't end the way as it was first presumed. It was Bevelle nor Zanarkand who won. It was a single person, Yevon, who ended it with the creation of Sin.