I'm looking for a fairy tale (ideal), myth or piece of folklore about a group of people or kingdoms forming an alliance to defeat some sort of evil. The ideal would be something about multiple armies coming together against an evil creature or army, but anything with this sort of motif of alliance-against-evil would be great.
I'm fairly aware of fairy tales collected by the Brothers Grimm, and as written by Perrault or Andersen. I've got more than a passing knowledge of Celtic (esp. Welsh), Norse and Greek mythologies. The closest I could really think of was King Arthur uniting Britain, but that's not really the line I'm looking for -- it's an alliance of equals, not a unification.
I have googled combinations of "fairy tale" plus "alliances" "war" "armies" and the like. Also tried things about fictional wars with multiple armies, which mostly got me things about the Battle of Five Armies from The Hobbit.
Any help or ideas will be greatly appreciated. If there's no folklore basis, I can just make it up, but the other parts of the story are heavily based in folklore and I'd rather have the overall arc.
Edit to add: Looks like we have a winner in the (Number Varies) Chinese brothers! Thank you so, so much to everyone for their help and suggestions!