Mance Rayder was once a Ranger of the Shadow Tower, but turned Wildling upon a disagreement with the Watch. Mance would spend a great deal of his life uniting the clans beyond the wall, and preparing them for an invasion of the South. His strength would peak in 299AC in which he led the Wildling forces in a Battle for Castle Black.
Mance was repelled and captured during this assault, thanks to the timely arrival of Robb Stark. Seeing as he was a brother of the Night's Watch once, Robb hands him over to Lord Commander Stannis Baratheon, who executes him. With his death, the wildling clans fell back beyond the Wall, completely dissolved of any unifying, leadership figure.