They started when the Old Alliance fell apart after a hundred years of peace. The states fragmented and Eldemar decided to attack its neighbors in order to regain control. The bigger states were expected to absorb the smaller ones, but instead they took massive casualties from the unexpected use of guns that overwhelmed the larger more potent mage armies.
Before the war guns had been considered mostly useless and couldn't compete with a trained archer, even less so a capable mage. The thing is, the technology behind guns had grown greatly from its early days where the weapons were very unsafe and hard to use. Even if a single gun couldn't compete with a trained soldier it didn't matter, firearms could be mass produced and used by an untrained militia and crush reckless mages with mass attacks, snipers and cannons. It took some time before the larger states realized they would also need to use firearms rather than just trained armies, but the damage had already been done by that point.
At the beginning of the wars it was considered prestigious for a House to have many battlemages and most of them decided to send their members to forefront. Due to this many great Houses took massive losses and were greatly reduced. The Weeping then followed and wiped out what was left of them, House Noveda being an example of this.