Senerii
The Senerii were a Spirit Valley Tribe who were destroyed at the Battle of Four Armies by the Fire Nation army. They were led by a king, known as Arkala, who preferred to die in glory than be exiled again or be forced to submit to the Fire Nation. A particularly warlike tribe, the Senerii army outnumbered the Fire Nation's host at the Battle of Four Armies and almost wiped out the Firelord's main force. This was prevented by the timely intervention of an army led by the leader of Even Newer Mexico, Bigby Branson, which took enormous casualties but saved the main army in the process.
Initially, Arkala had expressed an interest in allying with the Fire Nation. Initial emissaries were shot at by overzealous gate guards, however, which was interpreted as a sign of war. Arkala offered the Fire Nation a blood tribute in order to settle the matter and reopen negotiations, which was ignored. This was interpreted as a declaration of war. This interaction gives us some insight into the rationale of the Senerii - transactional, militaristic, and pragmatic.