So I was thinking there aren't many long standing alliances in warhammer. Historically everyone squabbles with everyone. The 3 chaos armies only really ally temporarily, when there is some exceptional leader like an Everchosen. High Elves only really take an interest in helping others when the the world is at stake, like a chaos incursion.
The only one I can think of is Empire and Dwarfs. That has been pretty solid for 2500 years.
Are there any others that people can think of?