I learned about the near extinction of north American bison in school but we were always lead to think it was accidental. That Europeans saw so many bison and figured they couldn't make a dent in that if they tried. Realizing that a lot of it was on purpose is so sickening.
To put this near extinction in a bit more context, I seem to remember reading a paper which suggests the biomass of hoofed animals like bison, deer, cattle, etc., was roughly the same pre-invasion of the Americas compared to today with mass scale animal agriculture.