Seasmy if you want the actual "history"
The Assassins and Templar knights (Knights of the church) have always opposed each other. Just because the Assassins fought against corruption and the knights took in polititians of the time, and did whatever they could to get to and stay at the top, meaning they would lie, cheat, kill... anything.
So the Assassins got together and pretty much wanted to stop them by eliminating the Templar knights as well as assassinate any corrupt officials.
EDIT: oh sweet
I like winning things lol, I'll try to think of a good achievement, will post it later when I come up with it