Metaphysical / Paranormal event
The status quo changed drastically after the monsters arrived. They looked human at the beginning. But the bullets didn't work on them. They lurked in the shadows and used strange powers to kill and subjugate the freedom fighters. Screams for mercy were answered with more bloodshed and red streams flowed through the cities.
Just like that, sparks of the civil war has died and United Clans of Cainite claimed its rightful place in history.