Apella was the leader of the faction that fought Tyree's people. The Klingons helped Apella by providing him with flintlock weapons which were more advanced than the bows and arrows that Tyree's people possessed.
Apella was being trained by the Klingon Krell in the use of the firearms. The Klingons wanted Apella to become the leader of Neural and a puppet of the Klingons. The Klingons offered him rewards and told him he would have power over the planet. They offered him more improvements to the weapons that would make them more accurate. Apella was also taught to reward those who did the most killing, which in turn would give his people more reason to follow his orders. Krell told Apella that he would make him a Klingon in no time.
The plot was discovered by Captain Kirk when he entered into Apella's village and overheard Apella and Krell. He was almost captured, but he and Dr. McCoy escaped after a fight with the pair. Kirk armed Tyree's people with similar weapons to keep the balance of power the same. (TOS: "A Private Little War")