A) Any historybook ever. There was never a point in history in which purple didn't try to slaughter green for green being green.
B) Everything. Me against my brother, me and my brother against my cousin. Me and my cousin against the world. Likeness could be anything that a person deems important. Race, religion, football teams, harry potter vs lotr, dota vs lol.
a) source?
b) what do you mean by "not like us"? in some ways, no human being on the planet is "like" me, in others, everyone is.