What lore do you ignore?

[Anonymoose]

Is it confirmed anywhere that disagreement over the third sacrifice caused an actual civil war and not just arguing?

Most English references say that an ideological divide gave way to open conflict and a final battle between Hydaelyn and Zodiark of a like usually found only in myth, but in French we have...

Hythlodaeus
Pour la première fois de son histoire, l'humanité était divisée en deux camps se livrant une guerre sans merci...
For the first time in its history, humanity was divided into two camps waging a merciless war...

I wouldn't say it was focused specifically on the third sacrifice, though. That was one part of a larger whole, the trend of trading more and more sacrifices (out of less and less true necessity) in a futile effort to restore "a past that could never be restored" (see: Unending Journey). It was a snowball at the core of which was the refusal to give new life and future generations a say in the way forward, aiming to reclaim the past even if it meant the "stewards of the planet" sacrificing the things they swore to be caretakers of.