It is later revealed that it was one of the leaders wife and mistress to the other leader. It was all part of her plan to overtake both groups or countries as the new leader.
The mistress/wife is stupid then. If both sides need an alliance, then there must be a threat they have to solve together. Or both sides must be evenly matched and have frosty relations to each other. Thinking that she isn't setting off a powder keg shows her inability to read the situation.