Title
What was your world’s crossing of the Rubicon?
The former
I guess the most recent one in my world was when a student at the Academy defected and began to plot against the realm. Sounds cliche, I know :p… she caused a lot of damage and used the Elders’ own knowledge against the them, forcing the Elders to separate their world from the humans’.
Mine is the Great Wyvern War, where wyverns became sick of humans over-hunting them to near extinction so they banded together to do the same. The eye-for-an-eye concept caught the attention of the dragons, who really didn't want humans to be extinct. This two-on-one war nearly destroyed everything as the magic users on the human side almost went too far and screwed up the environmental balance (you'd be surprised how integral wyverns are to the ecosystem).