And there were almost two decades of constant warfare+having to clean up the undead forces that were spawned during the SL.
When the Alliance acts as a whole we literally things like the Broken Isles where every member race is present in the push against the Legion, or heck seige of Ogrimmar. We have the end of Amirdrassil questing where all our leaders show up.
Hell, pre cata questline has ALL of our leaders meeting up and figuring out a plan to deal with it.
I would also point out the last time we helped get back the city of one of our member races, Gelbin was quite grateful:
“You never really understood, did you? It is our loyalty to our friends that provides our truest, greatest strength. This is what separates us from the ogres and troggs—and even the goblins. This is why the dwarves helped us back from near extinction, even surrendering a portion of their hallowed halls for us to call home. And this is why there are dwarves, humans, draenei, and night elves dying alongside us in the surrounding tunnels to reclaim a city that was never theirs. They are here because they are our friends, Sicco. My friends. It is a power that numbers cannot match.”