"SkyClan was the fifth Clan, but was forced to leave the forest because Twolegs had invaded and taken over their territory, turning it into the Twolegplace cats see now. Their current leader at that time, Cloudstar, appealed to the other Clan leaders at a Gathering and asked for them each to give up some of their territory for SkyClan. However, each Clan gave reasons to refuse to let SkyClan stay, and in the end, the Clan was driven out. " (Warriors Wiki)